3 mic's on a piano could be overly complicated. I would just place a stereo pair in xy configuration several feet away from the piano, facing the soundboard with the top open.
Don't be afraid to put a condenser in front of the sax, about a foot away.
You can use the 4th track for a dynamic close on the sax if you want to cover yourself, or just save it for cues.
These are "blind" suggestions, and you'll probably have to play around with positioning for a while, especially if the room is as live as most practice rooms I've seen. You might try pinning/taping some blankets on the walls to cut down annoying reflections.
I'm assuming you have neither time nor budget, so you probably won't be buying or building gobos. If you can find an old, used twin or full sized mattress it might help to get some separation between the sax and piano while enabling the players to still see each other. Or even a heavy blanket draped across a leveled boom could work.
Hope that helps, Good luck! [img]smile.gif[/img]