Okay first of all, don't feel bad about your assumptions. You realized that they were inaccurate and now know not to operate on those anymore. That's much different than the 'cowboy' philosophy that causes problems. Which is much more like trying to make the other person choose and choose you.
Right now, you are just discussing the possibility of a relationship. Dating and seeing what happens. There's no (or should be no) expectations of how you will end up. Maybe you decide later that it's not the kind of long term relationship you want. That's fine. As for giving the impression of a triad. No. I don't think meeting the husband will do that at all! As I may have mentioned, both of the men I am with are very straight. So straight that they both marvel at me 'going for them'. I'm pansexual and quite open.
There has NEVER been the expectation that someone I am dating will be involved with hubby! Typically if the couple is looking for someone like that, they put it right out in the beginning. So I wouldn't be overly concerned about it. Mention it, sure. Tell them you are new to all of this, ask questions, verify things. Don't assume!
I am constantly telling bf (also new to poly) to read up on things. I send him things here to read, as does hubby. We want him to ask, to verify, to understand. We want all of us to be on the same page and that only happens when we ask, admit to having a 'bad poly day', or that we need some extra space/assurances/cuddles.
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year