NO Ar-15 platform sold to the public will fire a three shot burst with one pull of the trigger,, or continue firing without stopping until empty or the trigger is released, unless the owner has a weapon that allows such. And for that he needs a Federal Class III license.
"Semi- auto" means the weapon uses part of the gas released when a cartridge is fired, to re-cycle itself, i.e eject the cartridge case from the fired round, and seat a live round in the chamber. No action by the shooter [use a lever, or a bolt, or break action] is required to recycle the weapon. There are myriad hunting rifles, aside from Ars that are semi-automatic. Ditto for shotguns.A whole class of handguns are semi-automatic [pistols as opposed to revolvers]. Hope that helps. 
When I served in the US Army we trained with both the M14 and M16, and both fired full-auto. We spent more time with the M16, and were warned that full-auto was not accurate and wasted ammo. They advised short bursts.
I read recently that the military sets current M16/AR15 to limit 3 round burst, but no full-auto. This consistent with training over 40 years ago.
Through the years, I've heard of modifications which convert single shot AR15 to full-auto.
In terms of legality, does it mean that any civilian possessing a full-auto gun is breaking existing laws? Are the conversions illegal?
Did the Colorado and Connecticut shooters kill all those people with one shot per trigger pull? IOW were their firearms "legal" configuration?
Do the proposed gun control masures require registered gun owners to turn in same?
*Both my wife and I are registered handgun owners in the state of California. The stuff on the internet would lead some to think this impossible.
My wife was held up by a guy with a shotgun, in the late 70s. He got sent to a facility for mentally ill. We understand back then they were frequently found to have miraculously rehabilitated and released.
The political left and right are both talking about the mentally ill and gun access. That does not address releasing them into the public. (just addresses keeping them from legal access to firerms)
A desparate drug addict/alcoholic is temprarily insane, too.
So law abiding folks need the ability to defend themselves from those who would do them harm, because the police are too slow.