My Monstera has three plants in one pot! Should I separate them, or leave them in one pot?

Short answer:

You can do either, depending entirely on what you'd like!

Long answer:

Separating them means that you can put each one on a moss pole (which will give the fastest and largest growth, with best fenestrations) and mean that you have three pots (which could be either good or bad, depending on your situation). Keeping them together, on the other hand means only one pot (which, again, could be good or bad) and that that pot will look "fuller".

Usually, pots with multiple plants in them tend to be very wide and horizontal; if you want them to be more vertical, you need a moss pole or support for it to climb, and that's definitely easiest/simplest and looks best with a single plant.