Okay, according to Complications on #capasb IRC, you ARE allowed to PM 3 and only use two:
<Unoriginal_Name> need clarification on this
<Objection> that's an old rule that some people do still follow but most don't
<Unoriginal_Name> thought so
<Objection> the rule basically says, if the match is a 1v1, 2v2 or 3v3, you PM 3 mons. otherwise you PM the mons for however big the match is
<Unoriginal_Name> so it's legal to pm 3 and only use 2?
<Objection> in a 2v2, yes
<Unoriginal_Name> okay, thanks
<Objection> it was designed to prevent unintentional counterteaming i reckon
<Objection> or at least mitigate it
So, you are allowed to PM 3. Once you send your second mon, the remaining one is void. You ARE allowed to use Squirtle, unless you use the other 2 before him. Thanks, and sorry for all this. :P