Agreed. I'd really love to see a Captain Marvel movie with Carol Danvers as the hero, but unfortunately I'm having a hard time seeing it happening before A3.
Assuming one new property a year, and if A3 is in Spring 2018, that's only two open slots (2016 and 2017). There are four potential new properties that I see talked about the most: Dr. Strange, Black Panther, Inhumans, and (Ms.) Captain Marvel. I suppose you could also add Black Widow and Runaways to this list, but the first I see being a one-shot, and the second I think is even lower on the list.
Dr. Strange seems all-but-confirmed at this point, and Feige seems much more excited about either Black Panther or Inhumans than about Captain Marvel. To be fair, all three of those offer something totally new to the MCU (magic, Wakanda, and inhumans), while Captain Marvel is sort of a mish-mash of previous elements.
They have the same problems with sequels: finishing up the Thor and Captain America threequels seems like the obvious move, but then there may be interest in creating a Hulk 2 and/or GotG2 sooner rather than later.
Even if Marvel could squeeze in more phase 3 films by extending the phase a year longer, or upping it to 3 films per year (or some combination thereof), I still think Carol's going to end up the odd one out.