Hey Ziggy, I love this custom stuff. When I use multiple engines, I try and keep them the same, same engine, same clutches, pinions etc. Then all ya gotta worry about is the tuning of the engine's to try and get them in "sync" with each other. Having different engines will cause you quite a few headaches!
Also having 3x 2speeds is gonna cause you alot more frustration.
I'm in the last steps of completing a similar project( 3x engines, not for speed records) and instead, I'm running 1x 2speed with single pinions and spurs on the extra 2 engines. You might have less problems doing it this way.
Just my 2cents worth.
Here's a pic so you can see what I'm on about.
(The 3rd engine wasn't on the chassis yet, but you can tell where it would go.