For those of you who do teambuilding, what do you look for in a 3-mon core? Just cover as many type weaknesses? Counter popular threats? Or do you just use a few guys that synergize ie. Tyranitar, excadrill and ferothorn?

I'm really bad at this shit; right now my core is Braviary, Infernape and Ferrothorn. I was trying to go for OU, but I also wanna use pokemon that I like. Any tips or suggestions would be great.