I'm in a MFF triad, and our 'rules' change all the time! It is nice of them to worry about your feelings so much but people do change over time. In my opinion one of the tests of a relationship is if all the people can grow and change (as they MUST) and still love each other.
I find that we go through ups and downs like any other couple. Sometimes two of us are more into it than the other, sometimes all 3 of us are raring to go! We try to just let things come naturally. Don't feel like coming to bed? Cool. We make it clear that there is an opportunity for sex and if someone bows out then there is no pressure. Not that there isn't guilt, we're still working on that part. This goes for activities other than sex too.
Do you go on dates in pairs? We didn't do that at first, but after about 9 months we started to do it to take advantage of our built in babysitter quality. I never realized how much we were missing! Its great to have that kind of time to connect. I know it sounds weird, but we bring a lot of positive energy back to the triad relationship by dating in couples. Perhaps your GF and BF could try that?