For what it's worth you can go back and look at what happened last season just by changing the drop down menu on your league page from Season 12 to Season 11. Doing so, it looks like only two teams promoted: the league champion and the best record in the other conference (which also happened to be the 2nd best record overall).
You're probably correct that the team with the best record in the other conference gets first dibs on bot promotion though: in
http://www.buzzerbeater.com/league/432/overview.aspx?seas... V.116 last season there were two promotions, the champion and the best record in the other conference, while the 2nd best regular season record overall, which belonged to a team in the champion's conference, didn't promote at all.
I wonder if anyone actually has a link to a conference that recently had three promotions? It may be that there just aren't enough bots for that to happen much anymore.