I've had both Martins cages and a double CN. I personally find the Martins to be much better. To the OP, the footprint is bigger than you'd think, and most rats rather enjoy climbing and having more levels. Despite being slightly smaller, the Martins 695 has far more floorspace than a single CN. Another consideration is what you will use for bedding. If you like liners then you can use a CN but if you like bedding then that is out unless you spend 65 on bass pans.
My biggest gripe with the CN was cleaning. Spot cleaning is easy, but thorough cleaning is not as much. They do not fit in the shower unless you disassemble it which is a PITA. They have no tools construction, so unless you have a perfectly level entrance/exit to your house with no drop or stairs, it would require you to lift the 100 pound cage and lifting it causes it to come undone. You DO need to either hose it off of disassemble it and clean it in the shower however, as where the frame comes together there is a teensy gap that pee gets gunked up in and it cannot come out without water pressure and scrubbing if you do not take the cage apart, meaning anyone who can't easily hose this thing off is going to have a heck of a time getting it fully clean.
Anyway as to the martins cage you find best, I like the 699 or RUDD because of all the space it offers. For three though, the 695 would be fine. Since you don't seem to like the floorspace though, they have ferret cages with larger floorspace. Only things are: you'd have to either get PVC coated and hope they don't chew, or custom order it powdercoated which costs an additional 10 percent since its a custom job, and the larger footprint cages only come with slideout pans, which I personally dislike. Definately check out their ferret cages though. Had I not found the 695 I found used, which meant I only had to buy a 680 to combine them into the RUDD, I would have gone with their ferret cages ( the ferret condo most likely.) Good luck choosing a cage!!
ETA: Someone mentioned their doors being small, they actually fixed that problem and their doors are huge now, so a flip top lid is no longer needed. They just seem to really suck about updating their pictures lol.