I'm having some trouble with https://scratch.mit.edu/projects/270803836/#player
When I press left/right arrow keys, I expect the square to go left/right by 1, but it goes left/right by 3. I don't know what has caused this problem.

Your problem lies within the movement code.
I believe its in the defined custom block, Sorry, cant help anymore, because scratchblocks take forever.

2 clones and the sprite are responding to the key press so that's 3 times a 1 unit change.

Oops, suddenly forgot that clones respond to key press. Thanks!