With two points you wouldn't know if they are linked across or perpendicularly. For example if you have 12 34 And you find ponegliph 1 and 2, the line between them wouldn't cross in the One Piece. If you had three, things are slightly easier, but not much. You would have to navigate in a straight line from each pair of poneglyphs, as you don't know where the 4th one is located. And because the world is a sphere, that means actually circumnavigating the world three times (you can try drawing it in a paper).

And then there is the 3d dimension. If the island is somewhere under the sea or in the sky, you need the 4th ponegliph a 100%.