You are generalizing things way too much. Poker is always situational and there are so many variables that nobody can come here and tell you with a few sentences, what are the top 3 biggest leaks of medium skilled tournament players all around the world.

Start working on your game and realize that poker is a very complicated game. Don't look for quick tips, because there are no such things.

lol i do work on my game..and sometimes it IS as simple as, these people do a,b,c consistently wrong...thanks for the helpful post though!

Also, the point of a forum post is to generate discussion, not "ask for tips."

Very dependent on buy-ins / locations. Pretty sure there is quite a difference between MTTs > 1K€ and MTTs below (even more when you take EPTs and the like versus 1K€ mtts) ; think also fields can vary grandly depending if you play in the US or in Italy....

As for answering more accurately your post, I'd say one of the biggest live leak in MTTs is giving off tells... So incredibly more than in cash game. Also players usually fit a category and don't really 'balance' / vary: that is, a tight conservative player will remain so, and will probably be un-spectacular, whereas an agressive player will probably have a hard time speeding down. Same thing online right? Expect in live games you can see sometimes at first glance who fits in whatever category, and even more after a few orbits: you can SEE who has his ego in play, who will have a hard time folding in general or vs you in particular, you can see who's playing his cards, etc. I mean, just the way people stack their chips. Just the way they throw it in the pot, how it varies depending on strengh / goals in the hand.
I won't give more infos on that because everybody has to work his ass off and focus on what he deemed appropriate, but I personally put a lot of weight on body langage / momentum in live MTTs.

As for onliners leaks when they play live mtts, the biggest is lack of patience / big ego.

I'd add that so many players are terrible after the flop, but that's also true of online MTTs, to a lesser extent. For a cash game player, it's quite amazing how so many players are very unbalanced postflop in MTTs.