If you ever plan to do anything about the heavy one, you will have to
start feeding them separately and go for meal-time feeding rather than
free-feeding. It isn't going to be easy... for either you or the cats.

Well, I don't know yet whether it's such a great food, but we got a
sample of Iam's new Multi-Cat formula at a pet show, and it's supposed
to address the problem of having fat and thin cats, old and young, etc.
in a household. So far all three of our cats like it, which is sort of
a miracle.  We cannot free feed because one of our cats would eat until
she explodes if we did, although this means that the thin cat who would
do better if she could graze all day long stays slimmer than we would
like. Whether the Multi-Cat formula well help control the weights of
the two pudgy cats while helping put on a few ounces for the thin cat
remains to be seen.

Another idea you might try, which worked for us when we only had the
older two cats--put a bowl of food up on a dresser or counter. In our
case, the fat cat could never jump that high to get it, but it was no
problem for the slim cat to jump up whenever she was hungry. It doesn't
work now because the third cat is too determined to get at food no
matter where it's placed and despite her pudginess she will find a way
to reach anything that smells like food.
-Yngver