Hey guys i was wondering what a decent party would be to defeat my friend, his main threat is a really strong feraligatr.

EDIT: Its a 3v3 not your usual 6v6.

It all depends on what other Pokemon your friend has, because if you try to beat him with six electric pokemon you'll take out his Feraligator easily but fall short against his other pokemon. Is that literally the only Pokemon he'll be using against you? If not, what else does he have?

Tyranitar and Curselax can handle almost anything he will throw.

for the third slot maybe Starmie? or Dragonite?

Well thats the only one i know about so the others are currently a mystery.

I'll probably be using a

Well type variety is very important, dual type Pokemon can give you and edge because they have access to more STAB moves and can cover each other's weakness easier.

A good team in GSC could consist of:
, and
, , and
, , and
, , and

This is not to say any of these guys are the best in GSC, but just an example of type combinations that result in no two pokemon in the team sharing a weakness.
As for honest suggestions:
Snorlax is awesome, teach him curse and other strong moves like rest and snore.
If legendaries are allowed, Zapdos is a strong Pokemon, and has a type combination that is unique and powerful.
Tyranitar/Dragonite, both of these guys do the same thing, deal but loads of damage and take hits quite well.

in a 3v3 you can't counter every type. the best strategy is to go with the strongest!