jealousjean, welcome to the forum. As the old saying goes, "be careful what you wish for", right? It sounds like you may be bisexual as you say you had a crush on this female co-worker for a while, and suggested a threesome in order to have a little fun and satisfy that curiosity.
What you didn't foresee - and didn't really want, by the sounds of it - was that by indulging this fantasy, a whole new three-way relationship was born. I think the most important question here is did you actually agree willingly to this situation? Or did you give into your husband (and the other woman's?) wishes under duress?
Now, you call it a "triad", and mention that your husband and your "third" are romantically involved with each other - but you don't really specify whether or not you and this woman are also/still romantically involved. Does SHE have romantic and/or sexual feelings for YOU, as well as for your husband? Do YOU for HER?
Are you and the woman also intimate with each other, or has this morphed into a situation where their NRE (new relationship energy) has taken over and you and your third have lost interest in each other, or your jealousy makes it difficult for you to be intimate with her in a non-begrudging fashion?
I may have missed it in your post, but do you three all live together? If so, that's going to make this even harder to unravel, if you're having serious second thoughts and wanting your husband to cool it with the gf.
It can't be overstated that communication is of utmost importance when it comes to polyamorous relationships. Do you know where you stand with your husband now? It doesn't sound like you do. HAVE his feelings for YOU changed or diminished, or is he just caught up in the whirlwind of NRE? Do you know WHY he was so keen to strike up an actual relationship with this person very quickly, instead of being content with a threesome or some "playtime" every now and again - since bringing her into the relationship was your idea, initially. Did they also know each other beforehand, or was the threesome the first time they'd met?
I don't expect you to answer all these questions here in the forum. They're meant to be rhetorical food for thought, and possibly the basis of questions you need to ask your husband. I think it's imperative you get to the bottom of what is actually happening here and where it is heading. I say this because you seem to be fairly passive in the situation, and you have to remember this is YOUR life too. Not only your husband's wishes and desires count, and you shouldn't accede to situations you find objectionable or intolerable simply to keep HIM happy if YOU are not.