The P80-SE is the oldest and is based on the original P80 design. It used the V6 analog ECU and in time will go away. It is still used in some of the Heli engines so that is why it continues to be available.
The P90-RXi is based on the P80 but with a bit more thrust and has an internal fuel pump and fuel valves, is kero start, and uses theV10 digital ECU.
The P100-RX uses the V10 digital ECU and Is the size of the no longer available P70. It is about 8 ounces lighter for smaller models.
Those are the basic differences.