I'm quite new to poly and I'm in a vee which will never be a triad. So our situations are very different.
But the couple I'm involved with have been poly for many years and have had a number of different kinds of relationships outside their marriage. They have remained stable and made it work. I think part of their success is the structure they have developed.
They don't have a rule-sheet or anything, but they keep things clear. As it's evolved right now, on date nights she sees me and he sees his girlfriend. Dates nights are usually Saturdays, although there's a certain amount of flexibility. Usually one date-couple has access to their house, and the other couple goes somewhere else. Etc.
The point is, it's clear what the boundaries are and who gets time when.
Anyway, you've been around poly couples and you know about this kind of stuff already. It sounds to me like if Alex agreed that you and Jenny would have a certain amount of couple-time he would not feel as left out (he would have agreed to the need, after all); and if there was an agreed-on triad day when you all did things together that would create a place-time to satisfy a need as well.
You know the drill.
Maybe instead of a possible homewrecker maybe you turn into the architect!