Welcome to our forum.
GalaGirl has already gave excellent advice, so I won't belabor that too much. The main points are to communicate a lot with your two guys, to find out what everyone is comfortable with, what everyone's wants and needs are, etc.
After all, people tend to be very unique and individual, and what works for one person is different than what works for another person. So communicate a lot, and find out what works for JS and JM.
You will probably also learn by experience. Something that all three of you see as a minor issue today, could turn out to be a major issue a week or a month or a year from now. So keep the communication going, and renegotiate the boundaries/rules as needed.
Keep reading on this site, and post your thoughts and questions as they come up. Other than that, just enjoy what you have here and now and be happy! Don't be pulled down too much with worries. You're very lucky, in that JS and JM have both agreed to give this poly thing a try. That says a lot for both of them. Not all men would be so brave.
I'm glad to have you with us.
Love means never having to say, "Put down that meat cleaver!"