If you want to talk about two apples and an orange, you would not use fruits, but pieces of fruit.

John ate three pieces of fruit; he must have been hungry.

If you want to talk about different kinds of fruit, you use fruits:

John's three favorite fruits are apples, bananas, and papayas.

If you want to say John ate two orange segments and a quarter of an apple, I don't know what you do:

John ate three pieces of pieces of fruit. *
John ate three slices of fruit. ??

If you want to talk about two apples and an orange, you would not use fruits, but pieces of fruit.

John ate three pieces of fruit; he must have been hungry.

If you want to talk about different kinds of fruit, you use fruits:

John's three favorite fruits are apples, bananas, and papayas.

If you want to say John ate two orange segments and a quarter of an apple, you can say

John ate three fruit pieces

(not pieces of pieces of fruit).