The federal government does not consider burst to be semi-auto. Semi-auto rifles are legal for civilians without any special license. Automatic weapons require a Class III license and no civilian can buy one manufactured after 1986. Three-round burst falls into the later category, and any gun store in the U.S. will use the same terminology as the ATF.
I've been in many gun shops, never heard them referred to as automatic weapons. At least in the midwest, I've heard them referred to as either "burst" or Class III weapons.
