Thanks for the replies everyone. I've been having some of the same concerns re: going out on "all three of us" dates and needing more of my own date nights with my husband. Even before he started dating her, I felt like we didn't have enough quality time together because he works three evenings a week 2pm-11pm, one of them Sunday. So there are two weekday evenings I spend alone and essentially one entire day on the weekends because we usually go out Saturday and sleep late Sunday. I have to get up early for work, so I'm usually dead tired by the time he gets home in the evenings. By the time you add in a day or two to go out with the new girl, and a day or two to spend apart with friends, there's not a lot left over for me each week. A lot of our "date nights" lately have been hanging out with friends, not the one-on-one time I would like. We've been working harder to carve out more time for just the two of us, but I'm having a hard time feeling like it's enough.
I've also been pushing him to organize something for all three of us, but he's been kind of procrastinating because he was still trying to feel out how much potential the two of them had together and wants my relationship with her to develop "organically." Or something. So we got a little five-minute intro last Friday before they left on a date and she is having a birthday party this weekend that I was invited to. I don't want to be a third wheel, but it's hard to accept that someone I don't even really know is having a huge affect on my life. But he has said she also expressed interest in other 3 person dates, so I know they will happen...just not as quickly as I would like.