Hullo and welcome!
Do a tag search on NRE. Distance helps me a lot; not just physical but also virtual distance. Lose his number for a while!
Ask your girlfriend if she would want you as her lover. For me, threesomes definitely work best with the 'each in their own turn' principle, not simultaneously everyone jumping each other. If she is only mildly bi-curious, making out might be just the extent of what she desires with another woman, though.
Me: bi female in my twenties