I found a way to combine a 3 star tribe with any tribe, not just another 3 star tribe. You need at least 2 3 star tribes, but once you do you can combine any tribe with one of those 2.

Originally posted by DraconicGuacamole

This isn’t a bug this is a feature. It’s is pretty well known, at least it used to be but it’s more forgotten these days because you can’t do it in multiplayer and it’s just a visual.