A 3-pole shunt trip will take up 4 spaces, just make sure you get a shunt trip that has a 120 volt coil or you will need a transformer for the voltage of the shunt coil.
It really doesn't matter which phase the control voltage is fed from unless you are talking about a 4-wire high leg delta which will have 208 volts on the "B" phase to neutral if so you can not use the "B" phase for any 120 volt loads, and you must make sure the oven does not have any 120 volt loads in it connected to the "B" phase, which being a 240 volt 3-phase rated oven I doubt it will since that voltage is only available in a regular delta or a high leg delta, a WYE is 208 volts which is more common.
By the way are you sure you have a 240 volt service? as if you do have a 240 volt high leg delta then the "B" phase should be orange not red?
