Some great advice already, but I would like to stress the biggest for me/us - TRUST. Agreeing on a whole bunch of things is all very well and good, but what you need is the knowledge that once something is agreed upon, it won't get violated when "lower brain" starts to try to take over.
It takes a certain amount of maturity to do this - and some practice - many of us (including me) have, I'm sure, screwed up on this front. For those that have, it makes you realise exactly what damage you have done. I know I vowed never to do something like that again, and I have stuck to it. It damaged the trust I had with my partner and that made things really, really difficult. And it was nobody's fault but my own.
Yes, he cheated on you, and that is a huge withdrawal for your emotional bank account with him. That needs work to build up, and in some ways, maybe him showing that he CAN do it differently with #3, going at a speed at which you are comfortable, will be a good opportunity for him to do that.
If he can't, then it's just going to damage your relationship even more. And this most certainly is NOT what most people's poly is about.
Please check out The Birdcage - an open, friendly Polyamory forum for all parts of New York State
"Listen, or your tongue will make you deaf." - Native American Proverb