Is this a new triad?

I'm glad the forum has been of help to you. You and your partners are making good progress, that's for sure.
Well. Last night Ben and I had a date. We went out for dinner and then sat in his car for a while before getting a hotel room. And I forgot to check back in with Sofia to let her know what was happening. Ooof, major mistake.

After a morning full of emotional text messages, we arrived at the conclusion that Ben and I are going to cool it for now. Sofia and I still have a lot to work on in our relationship and the added emotional weight of being metamours is not helping. And I committed to Sofia to be better about communicating and communicating clearly so that she can align her expectations to reality.
Sofia is having some jealousy feelings about you and Ben. It's like you should have a nice triad relationship, but Sofia is throwing a wet blanket over it. Also, she is doubting you when you say you love her and want to have sex with her. She is testing you, and the test is rigged. If you can't answer her "Why?" questions, then that supposedly proves that you aren't telling the truth. She surely knows that things are amiss in your marriage. Isn't that what a couple's therapy is for, is to address the problems in the marriage? Or is she on her way out, and doesn't want to be convinced to stay? I hope not, but that's kind of how she's acting.
I think the same; that Sofia is on her way out. Even if she doesn't want therapy, it might help you to go see a (poly) therapist yourself so that you can have that support whilst you are going through this rocky, crazy, lovely space. Big hugs.
Can I ask does that mean Ben and Sofia will also cool it for now too?
I haven't asked for that, and I don't really think it would help. I have at times considered asking but it never seemed like it would really be productive.

Sofia is on her way out
A little over a week ago I thought this too, but then we had a really good talk and it seemed like she was ready to work on our relationship. The latest incident may have put her back on the fence.

This morning she told Ben and me that she needed us to cool it or she would need to withdraw from romantic and sexual relationships with both of us to protect her own sanity. I mean, I get it. A month or two ago, I wasn't quite in exactly the same spot as she is now, but it was tough getting used to being in a relationship with someone who was dating someone else. She also made mistakes of under-communication at that time and it was really tough on me emotionally when that happened.
Sorry to hear that you have hit a roadblock. I hope cooling it with Ben will help you and Sofia work on your relationship (and communication) a bit.