If you don't mind the first bread being somewhat sweet, you can use it as a leaven. I have. I'm afraid I might not have any practal advice.
If I were in your situation, wanting to do what you are doing, I guess I'd separate it according to the instructions, make one friendship bread, boil some potatoes for a side dish, but really to have the potatoe water.....store the water in a crock and add a portion of the mix (really intended for a friend into the potatoe water (after it cools) then store it in a safe, dark place and give the other 2 starters away.
Its not really practal, unless you know you will continue with it. You can kill your batch real fast if the yeast doesn't get used and more potatoe water isn't re=added in a few days.
You'd have to be in a good routine. But if there's a will, there's a way. You got something cooking in your brain, and there is logic behind it. Go for it.
Now, keep in mind that the bread will have a completely different taste than what you might be used to; but it might not be a taste that you would mind. I can handle it. It will not keep for very long though. Bake and use. In 2 days, it will be even more sour than today. Thats my 2 cent's worth anyway.