There are several types of rubbing compound out there,does anybody know the differences of the 3m kinds,i just painted black charcoal and i need to rub it and sand it now.

If you're at the stage of wet sanding and then polishing, use "polishing" compound, not rubbing compound. Polishing compound has a finer grit in it.

What's the equivalent grit of rubbing compund?

it depends on which compound you are looking at. there are several different ones. the strongest "Rubbing compound" is a beige color with a white bottle i say its probably a 800-1000 grit. they go up from there, it should say on the bottle. or if you go to a good auto paint store they should tell you what compound you need for the job you are doing