Well, the 3 of them can work nice in ou, so no big deal really =)
So you want to start with milotic, right?, I rather talk to you on PO, we can set a date and time so we can work on this team together. But I can tell you in advance we can try two paths: the classic defensive milotic (which works best with hazards, as Liz already told you), or we can try a trickier and more gimmicky path that might work well as well :P
So milotic teams should be stall(classic)/balance(gimmicky), and gastro and cune on a rain offense team, does that sounds good to you?