Here you need to place three dots so that all dots turn green. If you imagine the dots to be eyes that can see horizontally, vertically and diagonally, you will realize that an eye turns green when it sees exactly four other eyes (they can see through other eyes, if many eyes lie in the same row, column or diagonal, they can all see each other).

Now you can do trial and error to get it right, but some thinking and contemplating will probably get you there faster, and is more fun.

Also note the vertical line just right of the hanging rope, it will be required to solve a future riddle, but is not relevant here.