Sorry. What a drag. Sounds like your good thing is over.
It looks like it's time for a serious dose of responsibility and enlightened self-interest on your part.
First things first: Maybe you can have a frank discussion with the young guy and have a LOOK at his recent std test results. (Don't take his word for it; he's already demonstrated a proclivity for deception.) If he can demonstrate that he has a clean bill of health, you have no worries, at least not on the health front.
Otherwise, you need to get tested, for your health and well-being. And your metamour and bf (her husband) need to be tested too.
Moving forward, you know now that the behavior of your metamour lacks honesty, integrity and respect. As does the behavior of the young man you are/were dating. Not polyamory at all, but just a variation on garden-variety cheating. Sorry.
The good news in this mess is that YOU haven't compromised your integrity. If it were me, I would put my cards on the table with all parties and then withdraw from the relationship. Your bf and metamour have serious work to do on their relationship to either repair or dissolve it, and I wouldn't want to be in the middle of that. Too much wailing and gnashing of teeth on all sides. And as for the young man, meh! Invest the time finding people who are worthy of you.