Put the condenser in the center, and the two Shures offset in an arc to make a semi circle in front of the singers.
Make the center mic the loudest and pan it dead center, then pan the others hard left and hard right for a spacial, stereo field.

Experiment with distance to see how the pickup pattern works in this specific situation.

Mic placement is a science all to itself - take your time and test out several places with headphones before you turn up the main system volume.

I have great success with choirs of almost 300 members using just two AKG C1000s mics. Place them at or above eye level too. Good luck!