It's not called class 3 anymore. It's just NFA weapons now. Class 3 is a C&R license I think now.
I'm a class 1 gunsmith and I work on a lot of machine guns. Mostly M-14's and BAR's. There are forms you will have to fill out and submit to transport them out of state like if you are going to a machine gun shoot at knob creek or sending it off to someone like me to work on. It's no big deal at all. Believe me the ATF will explain it to you when your application gets approved. They love all the questions I keep asking them because I'm the only one in this area that messes with them anymore. They always have to call another office to find the answers but they do find them and are always very nice about it. The ATF is not your enemy. They're just people doing a job.
When you get it, I suggest buying the heaviest safe you can get to keep it in. When I have them here I go to extremes in security but it's for good reason. Besides it being a huge investment for whatever customer that owns it I really do not want the local sheriffs department having to try tracking it down if it gets stolen. That would be scary. They have never said a word to me about openly carrying firearms around my farm here. Or even if I'm out on my horse riding. I'm never unarmed. I'm perfectly legal doing it but most people get hassled. I have good reason to carry openly.
I hope you don't live in Ca, NJ, NY or Chicago. Good luck if you do.