Ok I was on for a few hours tonight while the rule of three was in effect. I took a look around and noticed the following:
1. While the rule of three was in effect there were only 2 groups of merchants in the bazzar, mindflayers and elves. Both of these groups were groups of three. and there were a total of three people logged onto the server, Throbblefoot (as DM), 420, and myself.
This makes 3 groups of three from what are random instances. Ie #of players on the server, and #of random merchants in the bazzar.
2. when the rule of three ended, the type of merchants in the bazzar had changed. Now there were The elves, and human merchants. The humans were a group of 2.
3. Throughout the course of the night the rule of threes did not occur again while i was on the server, and I did not note that Mindflayers and elves were in the bazar at the same time again. Other combinations did occur but the event did not trigger.
4. I also made note of he other merchant groups that came and went wihle the rule was not active Red Wizards and Humans- groups of 2, Halflings- group of 4, and goblins- group of 5.
I am almost completly certain now that the mindflayer and elf merchants must be present for the event to trigger. But there may be another group of merchants that show up in threes that I did not record and may also be connected. So I'm pretty sure I have figured out how it works but I'll have to see it in action again before I know for certain.