The absolute easiest way is to use the Action Electronics P94 Dual ESC and rudder mixer.
See attached data sheet. Use the Lite version if the motors will not draw more than 10A.
The P94 also has an output for a third ESC for the centre motor.
Attached is also a Wiring diagram for the 'Hatchette' Bismarck suggested by Action Electronics' Dave Milbourn, who I note has also stumbled upon our humble web site😊👍
(Note the 'c' in the gent's name which was used for the ship😉)
It only uses the outer motors but adding the centre motor is easy with a small ESC connected to the output mentioned above.
I have the Revell 1
/350 kit and have bought the P94 Lite and will add an Mtroniks Marine 10 ESC for the centre motor. Hope this helps your mate get underway. Look forward to some pics / vids👍
Cheers, Doug 😎