Side note - just because your friends you spoke of date separately doesn't make it an open relationship instead of poly, I am definitely poly but I don't date people my husband does, or have group experiences of any sort (just incase you are saying that was what made them open instead of poly).
I think you need to have a talk with them or him, and explain you have no idea if you want to date both of them long term even if you're enjoying the sex. Explain what you think you are or aren't looking for, so you don't get caught up into thinking you need to be a triad, then having it fall apart if a few months down the road you find you've fallen in love with her but not him and you are resenting being with him when you really just want to be with her. Better to be upfront with the best and worse case scenarios sooner than later I think.
Now I fell in love with my first husband after three days so I know it can happen fast, but just because you're falling in love doesn't mean you need to worry so much about how everything is going to fit together this week!
Happiness will never come to those who fail to appreciate what they already have.