Oh, that's not too bad. You'll be having your contractor do wall fishes / wire runs for speakers, right?
You don't necessarily have to have all three speakers mounted on the same wall. What I've got in mind are mounts that swivel left and right as well as tilt up and down. You'll need that flexibility for toeing in your mains toward the seating area anyway.
Hopefully, unless there's a stud in the way, your contractor would be able to fish some
in-wall speaker wire and add a
binding post wall plate* wherever you want. Just use
a pair of these mounts for your left and right speakers if you plan to mount high, or
these clamping mounts if you can mount them low enough that the tweeter is close to ear level while sitting; then
one of these lower profile mounts for your center since vertical space is at a premium. Your right speaker can go on the side wall near the front corner and swivel to face the couch. The left, just put it where you currently have a picture hanging to the left of the doorway.
* If you'd like to pad your Monoprice order to $50 for free shipping, that's also a great place to get a subwoofer cable and HDMI cables if you need them.