We just fenced 5 acres of pasture not connected to our farm directly. It's about a 5 min walk from my barn to this pasture.
My arena is on my property and so is the barn. I have 3 horses. All 3 get ridden. I'm trying to figure out what to do with the others while I ride one. I don't know yet how they will behave when they cannot see the others. The pony will definitely need to see the others, but the two horses might be OK alone.
So I can either lead them back to the barn each day one by one (lots of time). Lead one back to ride and hope he will be OK alone. Ride up in the field where the pasture is and they can see eachother. This is a good option, but riding is limited to trails and hillwork mainly.
It's a good skill for your horses to learn to leave their pasture mates and work on their own. If you are riding all three, I'd walk out and bring all three up at the same time but if you are just riding one, go catch one, take it up the ring and ride it. If you have a particularly herd bound horse, your first week might consist of catching herd bound horses and then letting them fuss for a spell in a stall away from their friends before riding. Obviously keep yourself safe--don't ride alone if you are having issues, and don't hesitate to use a calming supplement to help a troublemaker to adjust.
I would think it would depend on your management at the moment. Do they all come back to the barn to eat, or are they out on the new 5 acre pasture all the time? If they come back to eat, just time your riding to that schedule maybe? For example, ride the hardest keeper first, so he's not riding on a full stomach, while the other two are eating. By time the first ride is over, the one who eats the least should be able to be ridden. Just an idea.
I think it probably won't be all that big a deal, after maybe the first time or two. I ride away from the other horses all the time, and everyone copes. I leave one home along, and they may run and call a bit, but will always be grazing peacefully when we get back. You can also ride one and pony one, if you want to exercise two at a time.