Check out a previous answer on repairing a vinyl liner tear we had here
It has the links to the supplies you will need and the procedure. I just wanted to add that it would be rather tricky to hold down a 3' long strip of a vinyl patch if it's underwater but, since it's on a bottom, perhaps you can prepare some weights you can position on top of the strip as soon as you apply it. Several weights (bricks come to mind) positioned along the strip will be needed regardless of whether there's water in the pool or not but will be crucial if there's water. If there's water, I imagine you may not even be able to do it on the first try but, fortunately, those repair kits come with approx. 80' of vinyl patch (about 6" wide), so you will have enough for another try, just in case.
Good luck!