When I was re-doing my bathrooms we discovered that 1 toilet had 2 wax rings and a second toilet had 3 wax rings. The plumber said he'd never seen 3 rings before and was surprised it hadn't leaked. The toilet with 2 rings had leaked and there was rot around the base that had to be repaired before re-installation.
The multiple wax ring approach is apparently frowned upon (I'm not a plumber so I don't have first hand knowledge). I understand it often leaks (and mine had). The preferred method is to replace the flange with a higher one (which is what we did) or add a plastic ring to raise the apparent top of the flange before putting on the wax ring.
