I think I understand what you are trying to do. You re pretty clear but I had to read it twice to catch on, and that's after plenty of coffee. That's on me, not you. You want a section of the floor to drop down and become the ramp. I personally don't like those openings in the floor, they feel awkward. You have to build a lip around it to keep bedding from falling out. That lip could mess up cleaning and access.
If the coop is 3' of of the ground, the ramp needs to be at least 5 feet long so the slope isn't too steep, even with cleats to help their feet get a grip. How many chicken are you talking about and how big do you plan to make the coop? I understand the concern of creating more space for your winters. Snow is not the only consideration, they don't like a cold wind either. And snow can blow in from the side. I think you should be looking at a wind block as well as a snow cover.
You probably plan on having that floor opening to be fairly narrow. Unless you have chickens that can't fly like Silkies or Frizzles, they can easily fly up three feet but they need enough room to be able to spread their wings and fly, You probably won't have that. If you decide to use an opening in the floor of the coop you might consider reducing the size and using steps instead of a ramp. They can easily hop up a stair made from cinder blocks.
Again, how big will it be? If you plan a walk-in, I'd personally want it on ground level, maybe even with a dirt floor. If it is a reach-in where you can't get yourself inside it probably should be elevated. My grow-out coop is elevated, I prefer a normal pop door in the side instead of underneath. Like I said that just feels awkward. I'd think you'd be better off covering at least part of the run, and don't forget wind block. I'm thinking more about your comfort and ease working out there. Your comfort and convenience is really important.
