I have 5 in my 125 and are fine for the most part (I'll catch a split second chase once in a blue moon). Pajamas are typically fine a group... some cardinals, like bengai, need to be one per tank or as bonded pair.
Mine are from ORA and were small when I added them all at once, so that may also have something to do with not having any problems. I'd think a group of 3 would be fine for your tank. Also note that they don't always hang out together and will be spread throughout the tank. When they are together though, I think they look awesome hovering in the open water above the rocks.