If you're bi, if his bi, if you like his friend, well go for it: It's party time

Since i'm hetero, the only threesome it'll be something like me+2 girls

Seriously, if you both feel confortable with that, i don't see any problem... well i do see a problem if you see that you've have more pleasure in a threesome than with you and your guy... that might be a problem in a near future....