2 ways to get clan xp. 

1) enlisting in the clan operations and playing the clan operations.  bronze gets you 250xp , silver 500xp, gold 1000xp, and first place 2500xp.

2) by recruiting members with founder status that have not used their founder status to clan boost another clan.  You get 500xp per founder.  Good luck trying to find a founder who hasnt used his founder clan xp.

Only premium members kills and captures will count towards clan operations.  There is no minimum count for a clan to gain clan xp.  However, getting gold with 3 members will be hard. Silver, can be done, bronze is easy, just play at least 1 game and get 1 kill, capture, or confirm and you get bronze.

Hope this helps you