how effective are the 3 round burst in CQB, and why are they(afaik) much rarely used than single firing until the target goes down? especially if you are in a room with multiple targets, isnt it faster to shoot a well aimed 3 round burst into the target and then switch to other target than doing it on single fire?

A "well aimed three round burst" is kind of an oxymoron.  With a controlled pair fired on semi-auto, you actually use (quick) sight pictures of the target before each shot.  With a three round burst, all three rounds are going regardless of where you (or recoil energy) points the muzzle.  Accuracy suffers, as a result.  When it's all said and done, controlled pairs do the same thing with better accuracy and precision.

 

Some weapons are offerd with a two-round burst function.  I think that is a fairly attractive option myself.