Ok. That's how you are on your end of it and where he has been in the past.
Where is he on his
end of it in the present -- NOW?
Again -- have you talked to him to know where he is in this process at the present time? Does he acknowledge and own his end of the deal at this time? Is he willing to work with you on mending the marriage at this time? Or is he just not emotionally present to do the work required of him at this time? Or you haven't gotten that far yet -- asking him these things at all?
It's kinda hard to figure out how to support you or give suggestions for what to try at this present time when I'm not sure what your support need is and what has already been tried at this time.
So I'll just offer general encouragement then. And say that I hope you continue to sort yourself out, and then later can sort the marriage things out with him.
But the triad idea? That's just adding more work to an already full load... so I'd suggest declining the offer. Everyone has a polysaturation point -- and at this time maybe one husband and one metamour (her as his lover) is all you want to deal with.