So in Mass Effect 3, progress in the multiplayer campaign carries over to your single player game. However, does having played a lot of multiplayer prevent you from seeing some "bad" endings you might otherwise see if your readiness is lower?
There is a decay of Galactic Readiness, so you could stop playing multiplayer for awhile and let it decay back down to 50% if you want to see the 'worst' ending.
That is one way to get "bad" endings. The other way is depending on how you play the solo campaign it is possible to not search any of the planets and do 0 of the side missions, this should give you a rather low military strength where you could use 100% and still get the "bad" endings.
I can not garuntee it, because I dont know how many troops you would get by doing it this way. But I have heard of some people that had so few troops for the final mission that every time they went to Cerberus HQ they got a message that mission failed without ever leaving the Normandy lol.