I got the piece of gong out of the crate by the sea, but how do I get it in place?
Do I have to wait for the other 2? or is there some special way to move it that I cannot see?
TIA

When you opened the crate and you had a villager pick it up, they carried it away and stored it for later. Once you solve a particular puzzle, that piece will show up along with any others you have found. If the crate is gone then you have nothing to worry about. It will show up sooner or later.

Did you clear the thorns and the access to the gong?
If yes, drop a villager on the piece of gong, he'll bring it on his place with the first one!