I'm someone with an interest in the three kingdoms, a civil war from 190-280 where the participants were supporting the Han or trying to replace it and would have considered themselves people of the Han. I also, due to the Han's collapse leading into it, some knowledge of the Latter Han in its decline. I will not go for things like Duan Jiong's campaign against the Qiang and all the animals he captured or Sima Yi massacre of Gongsun Yuan followers at Xiangping as I suspect your friend won't count it.

To be blunt, to say the Han-Chinese in three kingdoms didn't pillage, loot, seize women or massacre requires the records to be wrong. That the way the era was fought to be completely different from what the records tell us, the sayings of the time were wrong, the records of how some marriages came about were wrong.

Before the Civil War

In 189, the Han had been in a long decline, it had recently lost Liang and Bing provinces to Liang revolt and had to buy off Zhang Yan in Changshan. It was still united and at peace when Emperor Ling died and the latest round of eunuch vs gentry leaders (led by likes of Yuan Shao) began with the He clan split, Dowager He and He Miao seeing the eunuchs as a vital arm of the Han vs General-in-Chief He Jin who was persuaded by Yuan Shao that destroying eunuchs would ensure a great time.

During the talks between the two factions and the split family, He Jin was persuaded to make a show of force, summoning outside armies and sending his officers to gather more troops. Cavalry Commandant Ding Yuan, an experienced military man, was one of those sent out and he burnt the fortified town of Mengjin. The place was the guarding of the Meng crossing of the Yellow River north of Luoyang and the flames could be seen in the capital during the night.

When He Jin was assassinated, the angry Northern Army and the gentry leaders stormed the palace, Yuan Shu burning down the gates, Yuan Shao arranging the slaughter of eunuchs without mercy (and non-bearded men, flashing became a way of saving your own life) with death toll said to be over two thousand, as De Crespigny argues it was likely likely looted with the imperial seal missing for two years and the imperial family scattered.

This was the professional soldiers of the Northern Army during a time of peace in act of bloody revenge (and in gentry leaders case, political partisanship). Fire put on fire, the Han Emperor palace put on fire, (probably) looted and stormed.

Civil War

The records don't tend to tell us what happened to an individual city when it fell during the three kingdoms unless something was unusual or led to something unusual. It tends to only tell you something if there is a reason so, for example, a lot of individual biographies in the SGZ will never mention the figure eating. It assumes you know they needed food to survive and ate, food will only be brought up if it says something about their personality (they don't drink alcohol or they drink lots of it) or an incident occurs while eating.

Now the records talk about excess pillaging being a criticism about Lu Bu from the Yuan's, the records also offer other reasons but the idea isn't that pillaging was bad but Lu Bu and his troops were unrestrained. On the other hand, when Wu general Lu Meng seized Jing from Shu's Guan Yu while the latter was away in 219, Lu Meng's SGZ notes

"Méng entered and occupied the city, completely obtaining [Guān] Yǔ and the officers and soldiers’ families and dependents, all were comforted and consoled. He ordered that those in the army were not to harass the families, and not take from them. Méng’s banner man soldier, who was a Rǔnán man, took a commoner family’s one bamboo hat, to cover government armor, though government armor was public, Méng yet believed it to violate the military order, and could not for being from the same hometown yet abandon law, and therefore shedding tears beheaded him. Therefore in the army was shaken and afraid, and on the road did not pick up dropped things. Méng dawn to morning sent intimates to give relief to the old, asked what they did not have enough of, and to the ill gave medical care, to the hungry and cold he bestowed robes and grain. [Guān] Yǔ’s office’s stored wealth and treasures, all were sealed to wait for [Sūn] Quán to arrive."

It is noted because it was unusual, because it was part of a deliberate ploy by Lu Meng to isolate Guan Yu, to sway his army over to him. This was not "and here is the usual policy I will just state here but nowhere else", this was Lu Meng applying strict measures to stop usual behaviour.

Pillaging was a way to keep armies fed and rewarded when you didn't have the resources to support the army you needed. It was not unexpected and could be taken into account with advice, Xun Yu warning his local elders they would be pillaged and robbed if they stayed in such a vulnerable location, he moved, they got robbed and few survived. Or warning Cao Cao not to attack Xu while Lu Bu was around, despite Tao Qian's recent death, as Lu Bu would be able to steal the harvest and shake confidence

The war in Qing between Yuan Shao (via various representatives, ending with son Yuan Tan) and Gongsun Zan (via Tian Kai) saw so much plundering of each side that it was said there was not even a blade of grass as famine hit. Li Jue and his military junta pillaging would have a devastating (if possibly an exaggerated) effect after they beat back Ma Teng's attack

"At the time the Three Auxiliaries people were several hundred thousand households, [Lǐ] Jué and the rest released troops to rob and plunder, attacking Piāochéng​ town, the people were hungry and destitute, and within two years they were eating each other and completely wiped out."

Then there was a tactic of raiding, you might not have a strong enough army to invade and take land but you could raid into enemy lands, pillaging, taking manpower, animals and whatever else you could carry while raising questions about the ability of the attacked to protect their ability. Maybe even gain a wife as Zhang Fei captured his wife Lady Xiahou from raiding Wei lands (there were other prizes of war marriages from cities falling like Zhen to Cao Pi, Lady Du to Cao Cao despite Guan Yu's wish, Qiao's to Sun Ce and Zhou Yu). Tao Qian against Cao Cao before the Cao Song murder, Lu Bu leaving a path of destruction to destroy Yuan Shu's authority as a new Emperor, Liu Bei on Cao Cao a few times, Huang She against Sun forces are the ones that come straight to mind.

Others have rightly pointed to Dong Zhuo's destruction of the capital as he pulled the Emperor away from the potential reach of the coalition and of Cao Cao's second invasion of Tao Qian, notorious for the brutal destruction with Xun Yu later warning the people of Xu had not forgotten this.

Cao Cao was a man of established family with links to the long-established Xiahou clan. His troops would likely have been from Dong commandery, from Yan province, from Turbans of Qingzhou with three detachments from Yuan Shao of Ji including Zhu Ling. Tao Qian was from Danyang, his troops were the people of Xu and refugees from the civil war with reinforcements from Gongson Zan's Qing and You forces. Liu Bei did have some Wuhuan cavalry among his five thousand troops but the rest would have been and seen themselves as Han-Chinese. It is hard to see how this was not Han Chinese on Han Chinese.

Sources

Liu Bei's SGZ translated by Bill Cromwell

Various SGZ translated by Yang Zhengyuan

ZZTJ translated by Achilles Fang

He Jin HHS

Fire over Luoyang by Rafe De Crespigny

It would help if you could provide some more context, because an assertion like that is almost certainly going to hinge on some rather extreme notion of what a "sack" means.

For example, is the context something like the Mongol Invasion, or comparing military actions of the Qing to those of the Ming, or something else? Or is this a discussion about something like the Warring States or Three Kingdoms eras?