Were you to add two or three new sheep once you were down to one, they would not mix together as a flock, but the new ones would probably gang up on the original sheep. The reverse would happen if you added one new sheep to two remaining originals - the new one would not be accepted so would effectively be a miserable lone sheep.
I think what I would do would be to wait until you are down to two, then when one of the remaining sheep becomes frail, get them both put down together. I think that would be kinder than leaving them both to waste away and die naturally, with one doing so on her own. I would be most comfortable doing it that way, perhaps you would too. You probably have a couple of years at least to think about it, and if you know that's the plan then it's not too hard and you will be confident that you have done the right thing. Whatever you do, winter is a hard time for old sheep, so for timing get them put down at the end of a long lovely summer, before their winter struggle begins.