Ohm namashivaya,
Vanakkam,
I heard from elders that, single knot set of poonul is for Bramhacharies, and 2 knot for grahasta's. but the third one is not specifically for persons who doing "Apara kariyam" or "pitru kariyam" or any other things, it is required for persons who do "agni kariyam", still the main reason is , only grahastas can do the agni kariyam or homum kariyam and they should have the upper garments always, if not possible to maintain always, they should wear one more poonul, for the absence of it.
it not mean only at the time of kariyam, they should wear this always. But i saw few people, who not blessed with children wear only 2 knot set of poonul.
I request all our elder members, please give us the "Thath-prium" (reason/meaning/knowledge) behind these.. or (if not known exactly) please give us ur views, let the next generation like us learn from you, you are the only source for us to learn from,..... hmm, please,...