So for each and every shot, they have to pull the trigger? What makes it semi-automatic then?
If a rifle automatically loads a new round/'bullet' after the previous one is fired, it is a semi-automatic. If it also automatically fires when reloaded, then it is a full automatic rifle.
Can you fire a lot of shots in one round? Holy crap... and come to think of it, what the heck is a "round"? How does that work? And what the heck is a "magazine"?

I know less than a kindergartner, so please be gentle with this dummy!
A round = a bullet + casing, powder and primer. It is the full enchilada.
A magazine holds multiple rounds and generally uses a spring to push the next one up to be reloaded as the new loaded round.
And what is the power of the bullets out of an AR15 vs just a rifle? Cuz I read an article once by a doctor who works the ER and gunshot wounds and the doc said that these semi automatic Rifles tear the victim apart, and there are slim to no chance of survival if they are hit in an organ by them but if just a regular gun, the Docs have a much better chance of saving the victim even with a direct hit to an organ....???
High powered rounds tend to pass through muscle and skin without extra damage, and unless they hit an organ or a bone that makes it ricochet, it will be just a through and through wound which are generally minor.
A shotgun does much more damage, like my 12 gauge I have loaded with extra pellet 0-0 buckshot. That can cut a person in half if they are skinny, lol. I load for fat criminals, lol.
A 0-0 buckshoit pellet is about the same size as a 9mm bullet and my 12 gauge rounds throw about 12 down range. It could ruin someone's day, for sure.
In combat infantry teamms, there is generally only one full auto guy whose role is to defend the base line of fire, which is basically the line right in front of his own men and oversees a critical field of fire. For example, if you are trying to control a road, you would put your men on the highest ground near a turn in the road, and have the full auto guy posted at the curve in such a way that he is able to aim down the road and also clear the area immediately in front of his squad of men.
The full auto guy also carries lot of ammunition as he will go through it like mad. Even he has to control how much he uses his ammo so he wont run out or melt his barrel and warp it.
Everyone else should be on single shot fire or they will run out of ammo too quickly. IMO a three round burst is still too much, as the second round is usually off two to four inches, depending on distance to the target, and the third round is off a whole lot more, a foot or more, and is wasted ammo. You only use full auto if a lot of bad guys are trying to over run your position. But that is why you should have obstacles, trip wires and some sort of mine in front of your position, generally.