This means that the person has exactly three people watching you, not including themself. This is was originally by FBI, Secret Service, and Marines. But has recently been adapted to Thug slang. In the case of Thug slang, it means that the person has back up...

when you're hungover as shit after a spending a night drunk as a skunk

the 3 is katie lily and stella and they only have each other as friends

lily katie and stella

Having fatass fingers and then press e and 3 at the same time