Videoball works for 3v3, which fits OP's request.
Chambara isn't out yet on PC, but I've tested it at expos, and it's like Screencheat but better. Monochromatic characters in a world made up mostly of all-white and all-black elements, so you have to use the world to your advantage to hide and attack in an FPS 4p splitscreen deathmatch game. (You can press a button to black your screen out of you wanna mess with screencheaters.)
Blade Ballet debuted at PAX West this weekend. 4p top-down swordplay game. Great mechanics and creative battle arenas.
Death Squared is a 4P co-op puzzle game. Works with 2 or 3 as well. Really clever learn-as-you-go levels.
Has anyone mentioned Ultimate Chicken Horse? 4p competitive version of Super Mario Maker. Always a hit at my parties.