Does The Witness have a 3rd ending

The piece of paper gives you a solution that you can enter into the hexagon puzzle below the Windmill.

This is completely optional.

There is not really a point of no return you need to worry about. While there is a point that triggers the ending which you then can't abort, you'll be able to recover the savegame and complete the other puzzles (and I believe this is how the game is intended to be played).

When you do complete the game it will quit (on PC at least) and when you restart it, you'll start from a fresh savegame, but you can then enter the menu and load your previous savegame. That last (auto-)save will always be just before the actual point of no return. The OP said the game loaded that savegame automatically upon restarting the game, so maybe this was changed in a recent update. (Also, I vaguely remember from some YouTube video that the PS4 version will just present you with the loading menu right away instead of quitting the game.)

If you really want to know the "point of no return" that requires you to load the last save:

There's an elevator in the water, that is activated via two simple "puzzles" (they're just straight lines, I think). When you activate the elevator, then the ending sequence starts.

Some spoilers here, you might not want to read them until you've solved a lot more puzzles:

The same thing is true for the alternative ending. The "point of no return" here is actually a few minutes of gameplay worth before the actual end of the game, but when you load the savegame you'll still start from before that point. The next spoiler block contains that point.

When you solve the "hidden (environmental) puzzle" that involves the sun and the structure around the gate of light in the starting area, you open up the final area that leads to the alternative ending. This can't be undone, and you cannot leave the starting area once you do, unless you load that savegame.