Might as well go for it. You're 19 right? You're young and it doesn't seem you're in a committed relationship. It seems like you're only together for the sex. So it seems like having a 3some wouldn't be so complicated in your case. Now if you were in a committed relationship and truly loved each other I would think it through more. Bringing another sexual partner into a committed relationship can be tough with emotions, jealousy, etc.