My dh gets lots of jokes along the lines - of where do you get some peace & quiet in a house with 4 females? - even our gp says this. Dh can't u/s these comments. He just laughs, doesn't respond & moves converstaion along. Think that it's just an easy comment to make as sometimes people don't know what to say & it sort of breaks the ice. Good luck with your 3dd - for us everything is fine with 3.
IMHO the best answer to crass comments like "oh, but weren't you hoping for a boy" or "bet you want a boy for you this time" is a flat "er, no, actually". (But then I really really wanted another girl 3rd time round and nobody believed me so I am bitter and twisted!)
I go with motherinferiors comment about pre-empting them with how happy you are that it's another girl. Only the rudest of rude people could then pass comment. If that happens you are allowed by LAW to a) wallop them or b) tell them to f off - both are allowed if you are visibly pregnant, that way passers by won't think you're mental for hitting someone
Congrats by the way.