It really doesn't have to mean he wants her more than he wants you. It could just be that he so happens to be able to have sex when they so happen to have time alone. It really could just be very bad timing on the part of sex drive. My husband has a low sex drive as well. I worked nights for years. We'd go weeks without sex and then he'd just be in the mood and I'd be at work. We didn't have a gf at the time, but he'd watch porn and take care of himself-then have no sex drive for me for the next week. I felt less wanted than porn! But, after time, I did come to realize it wasn't about me at all or about how much he wants me. It was just the timing.
You have sex all together? Three way? Perhaps you could express that you've felt left out and ask that, if they are alone and sex comes up, they can call you in to share in that and you will give them more alone time on another night? Explain that you tie physical intimacy to emotional intimacy (I am assuming here judging by your writing) and when you aren't being physical as often as they are you feel emotionally left out as well. That isn't a bad thing. It is just in some people's nature. It is in mine. If this is only the first time she's been getting more sex it really could be bad timing. If this is constant, maybe there is a deeper issue that will have to be addressed later.
I'd rather be hated for who I am than loved for who I am not.