All you should have to do is get two Y connectors, one to split the throttle channel coming out of the receiver to the 3-in-1 and the brushless ESC, and one to split the battery to the ESC and 3-in-1. You still need the throttle signal going to the 3-in-1 because it is also used to control the tail motor. But the ESC will control your main motor instead of the 3-in-1.

On the Y connector that goes from the receiver to the ESC, remove the power pin (middle pin) from the end that plugs into the ESC. You need to do this because you already have a BEC from the 3-in-1 and you don't want the power from the ESC's BEC.

Look at a manual for the blade CP+ or CP Pro, the procedure should be very similar.