I can see where there's confusion - there's a lot of information.
a) The problem lay in S txting me to ask me if we had changed the parameters of my husband contacting her outside of our threesome. He had txted her out of the blue, despite co-creating boundaries around communications between him/her. It was innocent enough, but a direct violation of boundaries that we had established. There were also instances of him overstepping boundaries around trying to initiate one-on-one sex during sleepovers despite our boundary of only engaging in sex between the three of us. It lay in the disrespecting of boundaries/pushing of boundaries without good communication practices. It totally freaked me out, as I wasn't expecting that kind of emotional immaturity from him, and it put up a ton of red flags for me.
b) Responsibility and roles? I would say that there are 2 primaries - my husband and I, and we share a secondary partner. My responsibilities are to set up dates, keep lines of communication open and co-create a respectful, fun time for everyone. My husband's responsibilities are to co-create a respectful, fun time for everyone, be the driving masculine force in our sex life and co-create great dates for us - he also provides a lot of spoiling, massages and fantasy play. Our lover's responsibilities are to inform us if she is taking on additional partners, and to enjoy being spoiled.
c) Expectations for the future are unattached. In reality, there is no telling how it will all unfold; she is a law student returning home when she gets her degree (minimum of one year, maximum of two.) There are days where we romanticize about a future together, and we are open to a lot of possibilities (we have lived with, and shared a life with a third before, for example). As for boundary renegotiation - yes, totally, I am open to that, but if basic boundaries can't be respected in the beginning it makes me pretty wary - give a spoiled child their way, and you've got an even more spoiled child, you know?