Ok, searched for it and the only 3 ram diagramme I have is from the 75hp Mercury and it's so convoluted that your Seloc manual is probably more use. The manual I've got here is the Clymer Mercury 45-225hp 1972 to 1989 which SHOULD cover it, but doesn't.
The best bet would probably be to wire the trim system as a standalone system.
If you have a black, green and blue from your trim motor then black is earth,green is Down and Blue is Up. Wire the solenoids like a start solenoid supplying power to the leads-all nice and easy.
If you only have the 3 wires from the pump coming from the transom bracket there's no trim sender shown on any diagrammes so it probably doesn't have the facility. My later ('93) 50 merc didn't have the facility for one.
Hopefully these will help:-
Wiring diagramme for Commander control here(click)
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