This should be in NFA, not legal.
Yes, you can own a G18 IF there are any in the registry (there may be a few, but I don't think they are plentiful). The registry for machine guns was closed in 1986, so if it wasn't registered before then, it is not transferable to individuals. Because the supply is fixed, prices are extraordinarily high (a M16 starts around $15,000), and rare examples are typically more expensive if you can even find them (they're all used, so you have to wait for a current owner to want to sell his).
There is no "class 3 license". It is simply a transfer form that gets approved by the ATF for each weapon. If you can pass a background check for a pistol, you can buy a NFA weapon.