Well then, it seems you have gone from "open relationship" of just sex with others to "Poly relationship." It sounds like a whole new set of boundaries is needed. I think if I were in your position I would get about being as open and honest as I can with myself and in my communication and request some of my needs be met to clarify where I fit in and what their position is on one another. This should take care of most of your jealousy issues. Usually jealousy is fear or threat and needs being unmet. Figure out what the specifics are and then break it a part and deal with it bit by bit.
I don't think you can ask for it go much slower than it is now. What they have is perfectly reasonable for a budding relationship. Once a week sleep over, a couple of day time dates and texting every other day is not a lot and the usual in most poly relationships at the best of times. You could ask I guess, but it sounds like you mostly need to catch up on where they are at. It sounds like doing your own work, with their help, would be your best bet.
Have a look at some of the other threads on "jealousy" (do a search in the tags) and maybe this thread on lessons learned
and this thread on foundations of poly