I like #2 & 3 better because..even though #1 might show the dress better (ant that is what you're selling) #2 & 3 the dog looks more comfortable. The first pic while cute, she does look somewhat stiff, like she doesn't particularly like the dress and #2 & 3 she seems comfortable. I don't buy lots of clothes because I think my dog doesn't like to wear them (and he doesn't) and a pic of a dog that does look not happy to be in the outfit would just confirm my opinion and might make me think twice about buying it. But
after saying all that, I don't think people are looking at it as hard as we are in trying to give you our opinion, so probably any of the 3 would be fine. Because really all 3 are quite cute and anybody looking at any one of them would say "awwww, how cute" If you're paying for the ad, you should pick your fav..that's my opinion