No offence but do you actually play Karthus? Im quoting where you say "escaping as Kartus is really easy". It is. Unless your against Olaf, Fizz, Yi, etc. There are many ways for people to get past the wall - even if you land it - and Karthus goes down faster than a fat man on a cheap hooker. I do like the idea of 3v3 Karthus as I like 3v3 and I like Karthus, but I feel that the explanations you offer do not yet completely make it a viable option. Add more backup strategies, your role in the team (DPS AP CARRY), what doing that entails (HIDE), and EMPHASIZE how squishy you are. You don't really seem to be worried that the most played 3v3 champs are tryndamere (spin), Xin Zhao (dash) and Olaf (ragnarok) etc.

Put on "Require to Comment in order to vote" thingy.
I like this guide a lot, but add a section on how to play in in 3v3s. You only put how to use his skills and all that, which is good, but it can be expanded much more. Also, you have a typo . You said Requiem takes 4 seconds to channel. It takes 3 seconds to channel. Good guide though! +1