Mix flour, salt, and mayo in a medium-sized mixing bowl.
Add water a little bit at a time, mixing with your hands until the dough just comes together into a ball. Don't overwork the dough. Touch it just enough to form your ball.
Roll the dough out to a roughly 10" circle.
Transfer to a greased 9" pie pan.
Use as you would any unbaked pie crust.
Notes
You can use any full-fat mayonnaise if you don't care about the crust being vegan.