3rb is not as combat effective as a well trained operator using _FA_ (for break contact, room clearing, etc.).
semi-auto being the standard firing mode for US combat personnel, it does have some importance.
the minor difference in semi-auto trigger pulls in a 3rb rifle will not really impact the average combat use, or those who are not familiar with the system (or those who are not familiar with good match AR15 triggers).
IIRC, USMC shooting teams had taken to cycling the charging handle after each semi-auto shot to return the 3rb ratchet to the first notch and get a more consistent trigger pull.
if the trigger is released on a 3rb rifle before the full burst is completed, the rifle will not fire (as it should be, eh?). the ratchet will only have 2 or 1 notch/s left before the hammer is stopped from falling on the next 'burst.'
if you need more info, check with the guys over in the M16 Full Auto forum. i'll refrain from more posting, as this is getting off topic for this forum (Ammunition).