Hello. Actually, they're all correct! It seems like you already have a good idea of the usage of "no matter how/what". :) However, aside from that, yes, what Elena said is true, too, and the second sentence could also just be a general "fact": No matter how hard YOU (which also means ANYONE) work, you (THEY) can't earn 1 million dollars. In addition, in the first two you're talking about verbs ('work' and 'working'), but in the third one you're talking about an adjective (hard-working). Therefore, they're all grammatically correct. :)

The first 2 are correct, "you are working" means a period of time in present, "you work" means in general in present.