"Class 3 license" is a term that I hear thrown around a lot. Technically, such thing doesn't exist. Any individual that can legally own a firearm can legally own a machine gun as long as it is permitted in his/her state. No special license is required. What is required, is filling out the appropriate paperwork (Form 4 transfer), fingerprint card/photo, paying the $200 tax stamp, and submitting this to the ATF and waiting.
A type 1 FFL (regular dealer) can pay to obtain an 03 SOT classification. Then said dealer can deal in NFA items, like machine guns.
A type 7 FFL (manufacturer) can pay to obtain an 02 SOT classification which allows them to manufacture and deal in NFA items.
There are many more nuances, but I will leave it as this for the sake of simplification.