your link now 404's so I might have misinterpreted your original post.
I wasn't using physics for them, simply X - 3 every tick
You should check out the manual entry for Delta time ('dt')..
but anyway, have you tried doing the same thing to the player as to the platform?
when the player is on the platform, You can also have player X -3 every tick.
(but read about dt before you do that)