My 2.5-yr old has bowel movements on average once every 2-3 days. Should I be concerned?

No. As long as there are no other symptoms of pain or discomfort, that is probably normal for him.
Probably not. Infrequent bowel movement is not too uncommon in children and could simply be a habit. However, if child develops abdominal pain or if child's weight gain and lenear growth lag behind, medical attention should be sought.