We were in the exact same situation - 2 kids in Dionos and 1 in a booster. We install the two Dionos behind each front seat and leave the middle open, where the older kiddo can sit with his booster. The 2nd row seats slide forward to allow easy access to the 3rd row, especially for a child. There's no need for the seat to fold as it slides. I'll try to attach photos to show the space. As long as the child in the 3rd row can buckle themselves in, this seems easiest. The ease of access to the Dionos is awesome, compared to having one in the middle.
With that said, if one of the Dionos is rear-facing, that should fit in the middle with no problem and the 2nd row seat on the passenger side can slide and tilt. I tested to see if there was any way to get the Dionos next to each other, leaving the passenger-side seat open enough to slide/tilt. You could probably secure them in a way that wedged them together, leaving room for that seat to slide/tilt, but it would require moving the car seat on the driver's side closer to the door. I personally want them as far from the doors as I can, so I don't love that idea.
I went through all sorts of research and scenarios regarding this before confirming the Ascent, which we absolutely love so far. I tend to want to have the kids in the middle of the car, if at all possible, but I also didn't love the idea of a forward-facing seat in the middle second row. After seeing how far inset from the door the two car seats are, I feel more than comfortable about my kids' safety, and I'm borderline paranoid about that in vehicles.
Best of luck. Compared to all the other vehicles we tested (and, believe me, I drove my wife crazy with all the vehicles we tested), the Ascent just kept coming to the top. After several weeks of ownership, it's just a matter of it holding up. Awesome vehicle.