Those are definitely the elevator & rudder trim wheels. I don't believe the BT-13 had aileron trim. I think the inner wheel (closest to camera) controls elevator trim, with the outer controlling rudder...but I might have those reversed. Good luck!
EDIT: Here's an older pilot report. Trim wheels are mentioned on the second page.
http://airbum.com/pireps/PirepBT-13.html