A very recent post by /u/DanKensington on bias in the histories and how we work through that (with links) is well worth a read and I think would help.

How Truthful Is My Era

Pretty good.

It is never going to be 100%, my era was over 1,8000 years back for one. Over time, things slip through the gaps as things don't get recorded or things that were noted down get destroyed by the passage of time, we can't mind read so sometimes intent is not clear. It was often the men of a certain class writing the histories which provides a skewed perspective and influenced what got written while what didn't get covered. There are tropes, a bit of magic, errors, bias and yes, lies, propaganda and attack jobs

So why do I say pretty good? The main source for me era is the work of Chen Shou who served two of the three kingdoms, as a private project he gathered up information from each of the kingdoms records department and compiled it with commentary, organization and editing as required. It is well regarded, so much so one contemporary Xiahou Zhan (unfortunately) burnt his own work since it couldn't compete, and Chen Shou is considered fairly neutral under the circumstances. This doesn't mean it didn't have biases: he was a proud Yi native, a member of the gentry class, had friends (and rivals) and as a servant of the winning Jin dynasty he had to walk a careful line.

The Liu Song historian Pei Songzhi's annotations, collecting other works (including from those that lived through the era) and commentaries about the era and adding them as part of the texts, help fill holes and provide other viewpoints. There are other works that main intention is to cover other things (Jin, Han, Yi's history) but which also cross over into covering the three kingdoms era and some of its figures.

Historians are aware that, even with extremely well-recorded parts of history, a problem is... well people. A person may mean well but we are all influenced by those around us and our backgrounds, have our own agenda's and perspectives, even private history works had political pressures on. We take that into account, we don't just analyse the events but who was writing, in what circumstances they were writing, the trustworthiness of the writer, what bias do they have, what agenda there might be, what pressures they might have been under.

Even if I, in a cunning plan worthy of Baldrick, gathered all the AH historians and time-warped them (and somehow let us ignore all the problems there) to my era... well we might be able to fill holes in the gaps of knowledge but we might not all see eye to eye on incidents and even if we talked to a figure to solve a mystery about intent or plans, we can't mind read. After all, show people incident captured live on camera (like a sporting event), you might get entirely different viewpoints.

We learn a lot from, however fallible, the records. What people thought, the way people lived, who fought where, policy decisions, philosophical matters, medical advancement. Even lies help get us closer to knowledge, what were they trying to hide or spin can tell us quite a bit of what happened, their attitudes and perspectives at that time and what they felt they needed to show to the world.

For example "yes, I humbly gave up a title to appease this arrogant rival warlord, I'm so humble and he was so arrogant" tells us one leader (Cao Cao) gave up a title (General-in-chief) to another (Yuan Shao). So we have an event, then we can dig into the events around that, what was the context, what had recently gone on, what happened soon after, to see why one leader demoted himself. In this case, the warlord got control of the Emperor and got overexcited, overplayed his hand in charging his stronger ally with "faction" (effectively, treason)... which went down badly. So junior warlord Cao Cao had to make a series of gestures to calm the more powerful Yuan Shao down including demoting himself and giving that rank to the ally.

Two Sides of the Story

Huzzah, you have multiple sources about the war. They might contradict but it means more potential information so better than one source that might be consistent but is one source. They agree a battle has happened, that is fantastic even if one side might gloss over it and the other trumpet it, not all the details agree, or (as sometimes happens) "I won" "no I won".

Yes, both sides could well portray themselves as the good guy and the other is a traitor/villain. Some of that might be via memorial or proclamation where they are sending a message for the people, their officers, their allies and their foes, for the history books as well. It isn't perhaps really what the writers thought about themselves or their opponent but it tells us a lot about what tale they wanted to spin, what parts of their own history they felt needed explaining or justifying, what lines about the opponent they thought would work with the intended audience.

People like to believe they are on the right side, which means the one opposing them must be wrong. One takes that into account in analysing the texts, the bias. If both sides agree one side invaded the other, the result was agreed then yay. Some of the details might vary, a side may seek to downplay a defeat and not want to talk about it too much or put different emphasis (like an epidemic through the camp), the victor may wish to point to their skills and officers while bragging as much as they can.

What though if two sources disagree about a battle result or a deal made or who did what?, DanKengsington mentioned methods that might be used for finding out who won a battle in the post I linked it. It is that sort of digging through that has to happen to dig out the truth: looking at what other sources say (texts, letters, administrative figures, archaeological) or through the same sources (not unknown for a fib to be exposed elsewhere in a text), who is reliable and who (in this incident or generally) might not be trustworthy, looking at the actions before and after. If a warlord claims victory then soon afterwards rails against his general's competence and attitude in a stinging criticism, he might not have won and was trying to save face.

We might not have a 100% answer on some things, it could be we simply know that one side is fibbing and can build from there but we don't know the full truth to replace it because details are missing due to lack of sources and evidence. It could be that one can only make an educated judgement on a disagreement in the texts: based on what is known of the situation at the time, what happened afterwards and what we know of the people involved.