Hello, Lindsey,
Bob gave you a correct answer that both versions are possible:
(1) три белЫЕ тарелки;
And
(2) три белЫХ тарелки.

I would add, each version functions within a certain syntactic construction. The best explanation I could find is in the textbook by a famous Russian linguist, D. E. Rosenthal.

I will paste his explanation in Russian first so that your Russian penpals could learn it, too.
Then I will sum up his explanation in English.