Okay, after watching the replay for the first time... that was probably meant as a threatening gesture. Whether you want to call it a gang sign is up to you; I'm sure no one here thinks Pierce is in a gang or affiliated with one, but that doesn't mean he's not familiar with gestures. I couldn't tell you if that was or wasn't a sign of the Crips, the Bloods, or whoever else. However, to me it's pretty clear from the way Pierce was strutting with his "tough guy" pose, and the way he threw his hand, that that was meant as a threatening gesture, rather than a cocky one.
That doesn't look like it was meant as "three" anything to me. Of course, it's absolutely impossible to mind read Pierce's intent, but objectively, I can see where the "gang sign" people are coming from. I didn't want to weigh in until I could watch what happened a few more times, but having watched it again, those are my thoughts.