SC3 going cheap? Instant buy. Any fighting game is better with friends, of course.
As for story bits, missing the last few games might dull the significance of things like Siegfried and Nightmare being 2 different characters, but there are things on the internet you can read. Roto has the best one.
The gameplay is easy enough for a newcomer to play fairly effectively, and technical enough for more serious folks.
Get it, especially now that it's cheap.