I think most almost-3's are smarter than you are giving them credit
for. If she knows she can eat nothing but candy, of course she will
eat nothing but candy. If she knows that candy is a 'treat' and only
to be eaten as a supplement to other, healthier foods, she is capable
of understanding that. (And then choosing to either eat nothing, or eat
healthy foods followed by a small portion of candy.)

We have many kids at the preschool where I work who are about this age.
They usually have desserts packed in their lunches, and they know that
they can't eat the candy/cookie/whatever until they've eaten a
reasonable amount of their lunch.

Naomi