Are the caps glued on or just press-on test caps?
I'd take the caps off and pour water down "C" and see if there is a trap installed. Hopefully there is. I'm guessing that's for a tub/shower. "B" is probably for a toilet - after a 2x4 wall and drywall it will pretty much be 12" OC from the finished wall.
I would expect there to be a cleanout and a vent in the mix. If you eliminate the trapped fixture as a shower it's at least a start.
Edit - I'm going to guess that "A" is the vent. Don't take anything I say as the gospel though. It's fun figuring these puzzles out.