WOW what a minefield, eh?
Really, in a perfect world you'd sit down as a triple and have a really great counsel meeting and everyone would feel as though they are appreciated and you are all working toward the same goals and being valued and understood. We've had this sort of amazing meeting probably about 3 times in the five years or so that we've been having 'family meetings'! So ya, it's tough.

I would still advise trying and not waiting until it hits critical mass. For your relationship to be successful you need to all be on the same page in terms of goals and desires or you're working at odds with each other.

First off, reading between the lines you and your husband are looking for fidelity which your partner may or may not be interested in. I would like to point out that at the beginning of any romance it is natural to want a bit of exclusivity. The emotions can run all the way to outright green monster jealousy/possessivenes s. This seems counterintuitive to polyamory but it is just human nature. You are feeling stressed and unsure yourselves so you couch it in terms of "sexual health" concerns, "commitment issues" and various other things that sound very proper and correct. The thing is if you simply accept that the desire to bond closely to a new partner is natural and allow the feeling to exist on both sides it slowly changes into a deep trust and ability to grow.

I'll give an example: When I met Arch I was all over him like a harpy! We had an exclusive relationship wherein he and I were fluid bonded and I had no other lovers but he and Sigel. Arch was ok with this because it gave him some measure of comfort knowing I wouldn't find someone to replace him (silly man!) and since we couldn't all live together I had the fear of him realizing I wasn't what he wanted eased. It sounds mercenary and horrible but in reality it allowed us to be proud of our relationship and profess our love for each other in a very showy way. As the time went on we needed less and less reassurance that we loved each other and now if the opportunity should present itself we might be interested in having another person to play with. We have had exclusivity and time to build a strong foundation and relationship before we admit that it might be fun to contemplate another player...yes we are still on the edge of that wild frontier but we see it as being careful. What we have is amazing but can be very fragile! This was dang near the blueprint for my first relationship with Sigel as well...

Ok that all being said you should continue to tell your new partner that you care for her, might be coming to love her and want her to be happy. Keep in mind that you want her to be happy and have a place where she can grow as a person...and if that isn't with you guys then you will love and celebrate whatever comes for her. You should feel this way even for your husband!
You should definitely find a time before you all move somewhere you are depending on each other for survival and have a meeting where you express all your hopes, desires, goals and compromises you are willing to make at the present time (being open means being open to what might come your way, remember) and reach a consensus. Revisit this type of meeting often and let her know you are just as frightened as she is of what could be.

This will all help you lay some foundations for those times you will come into conflict...it will happen and your best defense is to already have a great communication system in place.

I wish you all the best of love and luck...if you need any advice or just want a cheering section (or shoulder to cry on) just drop me a note!