What if bitcoin miners are being used to generate rainbow tables for cracking password hashes.
No. They're not.
Anyway, try entering a wrong password more than 3 times on your online credit card or banking site.
See what happens.
What the OP was probably referring to is they crack the password hash that is stored in the dB. That way you don't need 3 tries, cause you get it on the first try.
I know this is unrelated to the thread, but OP, are you fontas by any chance? You know that your handle is "fontas" spelt backwards right?