The 3 round burst is most effective if you miss with the first round or there are plentiful targets.
The only way you can get it to track vertically is if you get lucky or you put a sand bag on top of the rifle to stabilize it.
We used to fire the M16(the fully automatic version)by making a loop in the sling, up towards the front sling eye, to hold onto, and using a bi-pod that clipped onto the barrel. You had to aim just like you would if you were firing on semi or you'd most likely miss with all 3 rounds, so the first round would hit the target, the other two would usually land 3 to 5 feet off to the sides or overshoot. At longer ranges anyway(200 meters and out).
Some of our weapons had a restriction device on them that prevented the selector switch from being placed on Auto, most guys would break them off, not because they wanted full auto capability, but because it would dig into your side while carrying the weapon slung on your shoulder or over your back.
Personally, I can fire one plenty fast enough on Semi, and a heck of a lot more accurately than I can on Auto.
But I digress,