Not sure if this is gonna help, but here's my experience. I'm also in a triad, however we all live together and sleep in the same bed. Believe it or not, sex with all of us works just like it did when it was just my wife and I. By that I mean, it happens when it happens.
We don't have a schedule or anything like that. It did tend to lean towards most fun times happening on the weekends because that was when we all had free time together. There were always romps that happened either first thing in the morning, or at night before bed, and sometimes it was all of us or any combination of two.
For us, having to schedule sex took something out of it. BUT, that's how things worked for us, that's in no way saying that it will be the same for you. As many have mentioned communication is the key. Have a talk about where he's coming from. Good luck!!
Live life to the fullest 'cause you never know if you're gonna wake up tomorrow!