ROTK is going to be the biggest movie because it is not rate "R" like Matrix and probably Terminator. It is proven that PG-13 movies make more money due to the younger audiences. That is why you won't see a rated R Star Wars. Even though they could and a lot of people would like it to be more grown up they just can't afford too. That is why ROTK is going to be the biggest movie in a nutshell.
HOVA
The Matrix Reloaded has already surpassed X2, and is still going. So, no hope for X2.
T3, no-way. It'll make SOME money, but nowhere near the most of the year.
I'd bet big bucks on Return of the King. Fellowship of the Ring got great grosses, and then The Two Towers did even better. I'd say it's a fair assumption that RotK will be the winner for this year. Though, Harry Potter very well may give it a run for it's money, it's between those two.