First, I would think that you will need to put all 3 on a schedule. Unless you have a palce that only Eliot can get to. Which I think would be highly unlikely.

I would split it up and do at least two feeding a day. I would feed the two 'plush' kitties there diet food and let Elliot have the normal food.

You have not been grossly over feeding them, they have been grossly overeating.

I know it doesn't look like alot, but trust me they will survive.