3-5-7 is a game I learned at Bark Lake when my home camp, Kenesserie Camp, sent me in 1990 for 18 days.
One of my leaders and I played 3-5-7 for hours during our overnight trip. We figured out the math of it and now I can win 99 times out of 100.
The basic rules are:
3-5-7 (Nim) is a mathematical game of strategy in which two players take turns removing objects from piles of 3, 5 or 7.
On each turn, a player must remove at least one object, and may remove any number of objects provided they all come from the same heap.
The object is to make your opponent pickup the last one.
There are other ways to play it which you can read about here.