It would be only 92928 - LD. If there were also a stent placed in the diagonals (which are branches), then you would code 92928 and 92929. But if only one stent is placed proximal, mid, and distal in the same artery, then you only bill for one stent. Code by major coronary artery, then branches..not by number of stents placed.
So, stent placed in proximal, mid, and distal LAD = 92928.
Stent placed in proximal and mid LAD, as well as stent placed in 1st diagonal = 92928 - LD, 92929.
HTH