Well, the quote
“We have a number of candidates from the comics and from the movies we’ve already made. It’s just a matter of finding the right storyline, the right filmmaker, the right time.” Someone in the audience yelled “She-Hulk!” Feige responded, “She-Hulk would be pretty great.”
So it seems that it's more based on the script and when they find someone with the right vision, and less about whether it fits into the Phase.
I don't think Ant-Man specifically "fits into what they're doing" for Phase Three--they were just waiting on when both the script and Edgar Wright were ready before they could start production. I honestly believe that if they had been in position to release it in November 2014 (and therefore have it be Phase Two, they would have, and it would have had nothing to do with whether it "fits into what they're doing" with the Phase.
All that said, I'd totally
be game for a female solo superhero movie that doesn't suck. I've been waiting for one for some time now!