1980’s mariner 60hp help

[Night205]

I have a 1980’s mariner 60hp outboard that I have inherited and I don’t know much about boat motors but am having issues with it. The issue I’m having is it seems to be cold blooded takes a while to warm up, it starts fine when in neutral but stalls out when engaging. As well only seems to start on two cylinders( lack of power) and will go for about 5 or 10 minutes then the third will kick in and boost power but that only last for a few minutes then drops power and then intermittently it will kick in but never stays at full power. Have checked plugs, plug wires, changed coils, checked the fuel, can anyone give me some insite on this motor