You said you weren't sure if she was coming, does that mean that she doesn't live with them?
If the three of them have a equitable relationship, all live together and will likely be seen together in future, then I'd want to explain it to my dc. But in advance, having thought a bit about what to say, and giving time for them to ask questions before we visited.
If it's a marriage with the third partner being a 'side' partner - living separately, less close and permanent, unlikely to attend family gatherings regularly - then I'd actually rather not have that conversation with my kids until they were older and more emotionally mature.
I have a polyamorous friend and it's an interesting philosophy, she explained it to me and I can certainly see the logic behind it, but I think there's more to it than just saying "some people love two people, isn't that great that they're happy". Those I've spoken to arrived at polyamory after lots of thinking and soul searching, it's not like a gay/ bisexual person who is that identity, it was a choice they made as fir them the pros outweighed the cons. I'd worry that a child who wasn't emotionally mature might draw the wrong conclusions from a hasty explanation. Maybe I'm not explaining myself very well. I'd rather have that conversation when my dc was mature enough to understand somewhat, rather than just be told stuff and nod, but actually feel confused.
Obviously if the polyamorous relationship is a long term committed triangle and they were going to meet both partners regularly then I'd explain sooner, but from your OP it sounds like a marriage between the two men and a secondary less committed third person.