Sora-
Sora likes New Years because of all the game events that happen around this time. Sora doesn't believe in superstition so he doesn't do anything at all. His New Years resolution is to beat twice as many games in half the time. He puases his gaming for like, a minute to give you a quick kiss on the lips before going back to his game. He'll play well into the next morning before pancaking himself on top of you and falling asleep.
Riku-
Riku kinda finds New Years stupid because it just wastes time, but it's important for keeping troop morale high so he allows it, just no fireworks. If you count superstition being that he locks himself in his room for hours on end and does nothing but working then he's got that all setup. There aren't that many options on what to drink so he just drinks cave water. His New Years Resolution is to win this war, just like it is every year. You'll bust into his room shouting 'HAPPY NEW YEAR' before kissing him on the cheek before running out, which makes him stop functioning for like, a minute before working again.
Tet-
Tet loves New Years, it's just so much fun! He does everything possible to try to have another good year and it's fun watching him prepare everything. Tet drinks ambrosia, since it is the stuff for gods but he adds a little special flavor to it and every year you have to try to guess which flavor it is (especially since he keeps it the same every year) and every year you fail. He'll kiss your nose as soon as the clock strikes 12 and giggle at how you react. You guys will party until you can't anymore and then some extra, New Years parties with Tet are always a blast.