Hi!

I delve into this quite extensively (from my old account) for the three main categories of crime in this comment, discussing the general idea of the trials as "victor's justice"

My overall conclusion is that

(...) we see elements of victor’s justice in that only the Axis powers were tried and that one of the three crimes, the Crime against peace, was arguably invented [In other words, applied ex post facto]. On the other hand, War crimes especially, but also CAH [Crimes Against Humanity], relies on established legal precedents and must be said to be fair and legitimate legal grounds for judgement.

It should be said that in my comment i only look at the general ideas of the crimes. The concrete case can use the law in an ex post facto way even if the crime has an existing legal framework, so my answer doesn't necessarily hold true for the concrete application in a case.

If you cba to read that long chain of comments (it is long) you can ask something more specific here and i can give a shorter more direct answer, or ask if you have further questions.