To answer your questions...
I am 36, he is 28, and she is 30. They have been together for six years.
He and I see each other maybe once or twice a week. He is an auditor for a bank and travels for work pretty often, which means I could go a couple of weeks without seeing him sometimes.
The three of us get together socially maybe once a month, and see each other at other events, too.
HMA, I can see why you would think that there is some manipulation going on here. But I can honestly say that he has been upfront with both his gf and myself about most things *except* when it comes to talking about his own feelings. This extends to any subject that he perceives as negative or self-pitying (like feeling jealousy). All three of us are actually just incredibly nice and good people, if I can say that about myself without sounding too conceited!
I agree with the advice of going slow and being cautious with how I approach things. I guess what I want to know at this point is: does it seem like I'm worrying too much about his gf? Should I be thinking more about what I'm getting out of this, and whether I'm happy? Or does any polyamorous situation automatically mean I have other feelings to consider, whether I'm directly involved with them or not?