All the rules other people seem to be having weird me out. 'You can't have sex with so and so, not for the time being, you can't kiss, no alone time, just threesomes, both are allowed one additional partner, always let me know if you are meeting up with someone, if you want to add another partner we need to discuss that...'
Not all of the above are rules, of course, guidelines maybe. People have those in place because of real issues. But I think having iron-clad rules set in place from the get go can really stop the situation from progressing beyond anything than weird power-games over who is having how much time with whom.
Maybe in poly time is the only thing left of which you can be legitimately jealous of? I don't mind getting involved with people who have relationship-specific rules/guidelines, but would balk at somebody trying to include me in them. Like the whole 'we can't date separately - you have to like/do/date/love both of us equally forever more'.
"Resentment destroys more alcoholics than anything else. From it stems the root of our spiritual disease."
"In dealing with resentments, we set them on paper - list people, institutions and principles with whom you are angry. Ask yourself why you are angry."
"In most cases it was found out that our self-esteem, our pocketbooks, our ambitions, our personal relationships, including sex, were hurt or threatened. We were sore, burnt-up." Alcoholics Anonymous, 64-65.