Well, look, what do you think would happen if she found out about everything before you were ready to tell her? How would she respond? Would she have a temporary freak-out that would ease down after awhile, or would she be like, "That's it! I quit! I'm leaving!"
Take that, and measure it against the chances (per certain amount of time) of "getting caught" ... remembering that virtually no secret will keep forever. Are things reasonably under control for where she's at, mentally and emotionally? Is there an exit strategy to ease out of the secrecy?
When we say, "She doesn't know," do we mean, "She does really know on a certain level, but she's just in denial about it," or, "She knows in a DADT type of a way and that's how she's comfortable with it." Is there anything she absolutely doesn't know at all? If she doesn't know, she can't give her consent. DADT can fly with a type of generalized consent, so that's fine.
If there was sexual abuse when she was younger, that's going to be a tough nut to crack. One thing that's coming to my mind is the thought about getting a poly-friendly counselor for her. A good one, one that she is comfortable with. Might take some research and shopping around. Sometimes an open-minded counselor is good enough if they're willing to learn about polyamory and incorporate it into their practice.
Basic jealousy can usually be managed:
Let us discuss the greeneye monster shall we?
How to slay the greeneyed beastie.
Jealousy, Envy, Insecurity, Etc.
How do you achieve compersion?
The Theory of Jealousy Management
The Practice of Jealousy Management
Jealousy and the Poly Family
Kathy Labriola: Unmasking the Green-Eyed Monster
Brené Brown: the Power of Vulnerability
But in her case, those links will probably only help her treat some of the surface problems. The stuff that's deep down will take a lot of research to get into.
If she is afraid of her husband leaving her, we know that's an irrational fear because there's no empirical evidence that it would be likely to happen. And she may be well aware of that too, but sometimes the mind can say, "I get this," while the heart says, "I don't get this at all." Emotions are like fire. They aren't known to check in with logic to see if they should burn. They just catch on whatever kindling is available.
If she is a people pleaser, she will have an extra hard time opening up, because she will be afraid that her truth will displease the person she's talking to. So, lots of patient encouragement, and sometimes you just need to sit down with someone and share their quietness. Maybe a hug is all the communication a person is ready to receive at that moment.
All three of you can practice communication techniques and work toward building the kind of trust in her that will allow her to open up. Right now I think all you're hearing is the voice of her insecurities. The voice of the source of her insecurities is silenced.