Hi Barb and welcome!
I’ll start by saying I’m learning here with you too, but I can share some thoughts and try to work through it with you. Depending on the labs and phosphorus levels, I’ve seen some caregivers with early stage cats focus on slightly lower phosphorus wet foods, hydration, and keeping the cat eating well overall. Which would probably be good for all three of them. From what I've read so far, it seems like protein restriction in early stages is somewhat debated. Which makes sense to me. You don't want them losing muscle mass early, but also while their appetite is good in the early stages maybe there is a benefit to introducing the kidney foods. But then you're back to the three cat dilemma. If you haven't seen, there are some food charts
here that list calories, phosphorus, etc.
For hydration, with my stage 2 guy I add a decent bit of water to his wet food and have a fountain and bowl near his favorite hang out spots.
If you do end up separating their meals, some folks use microchip feeders if that's an option.
Hopefully you get some input from the other members here too. I would be curious to see the bloodwork and what food they're eating now.