Here is my 2 cents...
Do you expect there to be a transition time / getting use to things time? I think you could. You have been dating 3 mos.
You could thank her for telling you and let it go for now. She told.
Yes, the agreement is to tell WITHOUT you prompting but you could call this "training wheels time" to help her GET there. Esp since she's struggling with this:
The "scared" part is because she still feels a bit guilty being with other people even though we clarified a hundred times that it is OK.)
Saying something 100 times is not experiencing "ok-ness" 100 times. For her to believe it is truly ok she needs to have both. You
saying it is ok and the that "it is really ok"
experience with you and not experience you having GRRR with it.
You seem to understand that your response can influence future forthcomings. Like if you wig out over it now, she will be more hesitant to be up front or tempted to lie to not have to deal with your response. Since you want to have her be up front and want to create that climate? Focus on creating that climate. That part you can do. Wait for her to meet you half way. It takes time for her to get over fear. That part is her job. You cannot do it for her, you can only encourage.
Instead of focusing on what did not happen (instant desired OUTCOME!) you could focus on (what progress toward desired outcome from last time?) instead. Baby steps.
She was honest when you prompted, so keep checking in and ask. A weekly check-in is plenty. Not after every party, every day, etc. RELAX.
For managing your OWN transition "nrrrghs! Why isn't this faster?!" come here to vent, not your partner.
Comfort IN, kvetch OUT.
You have only been seeing this person for a short while and in your other post you talk about hoping it will STAY open. Help that happen with your behavior.
HTH!
Galagirl