derives from holding up three fingers to form a W.
The W usually represents the slang term "Word" and sometimes "Whatever". "Threes" can also be used when someone says something stupid and you reply "What?" but instead, hold up three fingers and say, "Threes."
For example, "threes" can be used in situations like these:
Friend: "I'll meet you at the dining hall at 3, I have class now."
You: "She just cheated on me with that dude. But it's okay I'm move on and I'm gonna get with her best friend. Threes Bitch!"
Friend: "My dog is black."
You: (must hold up three fingers in this case) "Threes to you making that dumbass remark."