I'm with TimOBrien. See if you can borrow a local church or something for a few hours, set up like a performance area and set up the Zoom in front and slightly above the trio so the X-Y configuration of the mics is just covering you from side to side.
Failing that, the advice above--to walk the room and see where things sound best then put the recorder there--is about the only way. When finding the place, you'll be amazed at the differences moving your head a few inches can make.