With the notable exception of Zhuge Liang's propensity for cooking thousands of enemy soldiers at a time (which appears to be uncommon), the regular soldiers are frequently well fed and given new uniforms.
Most engagements are described as duels between opposing officers, and the men following whichever officer gained the upper hand would summarily rout their opponents. These routs generally are described as resulting in the capture -- and subsequent enlistment -- of the erstwhile enemy army.
The common soldier seems not to have much say/care for who gives the orders, so long as they're fed.
I understand that the book is romanticized fiction, but it's lead me to wonder if internal conflicts in ancient China were comparatively bloodless with respect to Greek, Roman, and European campaigns (to say nothing of the endless bloodbath of Slavic history).
It is difficult to get any sort of hard numbers or probabilities in place, but I think we can say pretty definitively that being a peasant-turned-soldier in the Three Kingdoms era was not such a rosy life as you might expect from reading *Romance of the Three Kingdoms *(Sanguo Yanyi).
Three Kingdoms armies were largely made up from press-ganged peasants, not volunteers, and they were neither trained nor experienced. A big part of an officer's duties was to maintain discipline and morale, because once the army lost cohesion or began to act as a mob it was extremely difficult for the small cadre of officers to regain control of thousands of soldiers. Maintaining discipline was usually accomplished more through fear (e.g. executing attempted deserters, flogging lesser misbehaviours) than through inspiring loyalty, though letting the army plunder a conquered town could significantly bolster morale and loyalty.
I don't know anything specific about the food situation. It was extremely important for commanders to keep their armies fed since hunger quickly plummeted morale, but that doesn't mean the food was any better in terms of variety or quantity than the average contemporary farmer's lifestyle.
Uniforms and equipment? These were the exception, rather than the rule. For the most part, armies simply used banners for visual cohesion, and peasant-soldiers were only equipped with their basic weaponry, not with uniforms or armour.
Most engagements are described as duels between opposing officers, and the men following whichever officer gained the upper hand would summarily rout their opponents.
Individual duels or direct confrontations between officers on the battlefield did indeed occur, but the novel would suggest that these were more common than they really were, that they would usually happen right at the start of a battle, and that once one general slew the other the battle would end almost immediately. There's a hint of truth to this, in that generals of the Three Kingdoms period did indeed lead from the front and would charge through the enemy armies, but they weren't doing so alone nor were they doing so to try and find the enemy general for a duel.
Since both opposing armies in a Three Kingdoms battle were typically basically armed peasant mobs, the order and cohesion of the enemy army was brittle and the best way to "win" the battle was to break up the enemy formation with frightening force (before the enemy can do the same to your brittle mob). High-ranking officers would have actual military and combat training, and would also employ a small personal force of trained fighters/professional soldiers. They and their personal force would drive into the enemy army, easily defeating the peasant-soldiers in their way by dint of superior training, experience, and equipment. Furthermore, your own peasant mob would be much more motivated and effective if they followed behind the charge of a leader and his band of professionals than if just told to charge blindly towards the enemy. Eventually, one army will have taken enough losses and be in enough logistical chaos that mass fear will overwhelm the command staff's ability to enforce discipline, and indeed the army will be routed.
If one officer and their personal force charging through the battlefield ran into another officer and their force, they could certainly clash in a "duel" of sorts. Once one officer and his staff died, this would definitely be a cause for panic amongst that officer's surrounding soldiers, too, but it takes time for panic to spread across an army of ten thousand men, plus there are other officer forces amongst the army, too, so it's not like this would immediately trigger a rout through the entire enemy force.
What happens to the peasant-soldiers who fled during the rout? Well, they might soon be rallied back into a fighting force by their officers (as Zhang Ge did against Liu Bei in Hanzhong), they might flee into remote areas and become bandits, they might try to return home, they might flee in any which direction with no major aim except survival, or indeed they might surrender to the opposing force.
In the latter case, yes indeed as you say they might be enlisted into that opposing force. Or they might be pardoned and sent home by the opposing commander. Or they might be executed en masse, as Cao Cao did to Yuan Shao's surrendered troops after the battle of Guan Du.
So, in conclusion, I think it is possible that a peasant conscripted into becoming a soldier lives a "better" or "easier" life. Perhaps his farm was already failed and he had no remaining family or positive relationships, and then in the army he receives better food than the little he had at home, he falls mutually in love with a camp follower, his army is always victorious and he's never in the front ranks so rarely does any major fighting, he finds some great spoils while plundering an enemy town, etc...
But I'd consider that situation to be less likely than that his army's supply line is raided and he starves, or he is trampled when his army is routed, or he dies of sickness on the march, or he tries to flee and is executed, or in his first battle he is easily cut down by the personal guard of Cheng Pu charging into his army.
All that being said, I did a bit of searching and couldn't find any hint of a good source for actual military casualty rates from the Later Han/Three Kingdoms period, which would be a very interesting topic to learn more about. No, the battles were not nearly as bloodless as the later Tang operatic tradition and novelization describe, but just how many soldiers did die in the average battle before one army was routed and fled?
My main sources for the above would be the works of Rafe de Crespigny, especially Generals of the South, The Northern Frontier of Later Han, and his commentaries to Sanguo Yanyi.
The Yuan Shao chapter of Sanguozhi (by Chen Shou) for Cao Cao's executions at Guan Du.