One unstated rule is that you have to use all pieces. Number three can already be solved with two pieces and that also doesn't count. I strongly dislike this kind of puzzle.

Thanks for sharing. Where does it even say you need to use every piece?!

An unstated rule: not said/stated.

It doesn't say it. It probably should.

That also happens on an earlier level. The answer appears to be that there is an undocumented requirement to use all of the tiles. I'm not sure why Google appears to believe that documentation is weakness.

There's even documentation. Clicking on the (i) icon says "Place tiles on the grid to create a path for your marble". It would be so easy to just change it to "Place all tiles..."

Because at Google, you're expected to spend your entire budget

Hmm, I got the solution it wanted after a lot of trial and error, but I still don't understand why that one was correct