"class 3/III license" doesn't exist.
ffl, either type 01, 07, or 02, allows buyin' & sellin' regular non machineguns.
apply for ffl, fingerprints/pictures/copy of application sent to CLEO for INFORMATIONAL PURPOSES ONLY, payment of $200 for 3years as FFL. After that is approved (subject to local zoning regs for any FFL) then Payment of the annual Special Occupational Tax (SOT) enables the dealer/mfg to do business in machineguns/suppressors/etc. FFL accepts permiting ATF to enter place of business (licen sed premise) un-unannounced once per year during normal busines hours as noted on inital ffl application for "Compliance Inspection". ATF can look at the bound book and compare it to inventory on hand, and compare both to any MG/Suppressor/etc inventory show in the ATF NFRTR.
ATF cannot drop in anytime they like without probable cause, usually sufficient to rate a warrant. they can call & request info on firearms other mfg's/distributors/ffl's have records showing said firarm was shipped to ffl of interest.