I’m not-not against a threesome?

CONTEXT - My partner (he/they 21 switch) and myself (she/her 21 switch) have been together for soon to be 2 years. They have always been pan and myself straight. I have had three other intimate partners but I am their first.

My few previous partners were straight males who were dominant or new to non-vanilla bedroom activities. My current partner is male but more feminine and more submissive leaning. Honestly, it’s nice to get away from being the one without control after so long.

During the beginning months of talking/going into dating and understanding they were pan and I encouraged a more open relationship so they could experiment. They said no and I was fine with that. I have no interest in finding any other romantic or sexual partner long term outside of them and prefer monogamy in the end. This is something we have discussed many times and agree on.

Now recently they brought back up an open/poly/threesome and at first I said no but I once again find myself not wanting to restrict their exploration. We had a long conversation about what it would entail and what insecurities and boundaries may be addressed. For now we put the conversation on hold so my partner can really think about it.

TLDR- I am my pansexual partners only intimate experience and I wish for them to self explore. In the perusal of a threesome I have no interest in being romantic or sexual with others long term. I only want my partner but we don’t want it to be a cucking. We agreed the most that would happen would most likely be a submissive/switch third who tends to my partner. I could lead both of them but we agree a strictly dom is out of the question and I am more than okay with that.

Does anyone have any advise when getting into a threesome/odd partnership? How to make sure that even if I don’t want a connection with the other person they still feel comfortable and well cared for? We intend to communicate every step of the way and not jump in until ready.

(Posting to other communities as well)