I don't see why it would be no good simply because it's been in the fermenter for 3 months, but the best way to find out would be to draw a sample and test. You should probably do that anyway just to see what its FG is.
I doubt you really need to pitch more yeast for bottling, depending on the magnitude of the stout. If it wasn't too large (i.e., <1.070+), I wouldn't worry about it. It may be slower to carbonate than if you'd bottled earlier, but it'll get there eventually.