Here's a schematic which involves seperating the receiver from the 3-in-1, and using extensions to connect to the brushless ESC and the rudder to the 3-in-1.
Cutting the red wire in the extension is the easiest way to seperate the brushless ESC from the tail motor ESC in the 3-in-1.
For a different receiver, remove the Eflite receiver and plug the new one in using this same scheme..
Note you will have to check the main rotor rotation after assembly. Swapping any two wires from the ESC to the motor will change its direction of rotation.