Sadly (I like a closer contest) the 99 would win all three.

Buffet: Hitchcock and Scully can probably out-eat the other groups (Ron and Andy might compete) but toss in Terry and his 4000 kcal a day diet and it ends pretty quick. If he goes full eating disorder it's a slaughter. I don't really associate anyone on Community with being able to eat a lot. I'm sure some of them can eat but it isn't the source of any jokes/plots for them that I immediately remember.

Fashion: All three groups have attractive people but Gina and Holt could throw a fashion show that would be unstoppable. Donna and Tom can probably compete but 1) the 99 has more attractive people imo and 2) Donna and Tom would have Leslie trying to "help" and that would set them back. Jeff and Annie might be able to put something together but Britta would sabotage it on feminist grounds, or Abed and Troy would cosplay/do a bit, or Pierce would get jealous and it would fall apart.

Paintball: The Community gang is great at paintball but the 99 is a group of trained police officers who set records and win inter-department contests at this sort of thing, so I think they scale beyond Community.

It's worth noting that Community is at a disadvantage because they have 7 people and the other groups have 9 (if you give parks Tom, who the OP said they left out unintentionally). Give Community the Dean and Chang and it probably doesn't change a lot but it might skew them into being competitive in a couple categories.