Is it true that an apple, onion, and a potato all taste the same if you plug your nose

Yes

Explanation:

It is frequently quoted that upwards of 80% of our taste is made up by smell. So if you plug your nose and cover your eyes, the taste between an apple and onion should be indistinguishable. The logic makes sense ... All potato chip flavors taste the same.

This is true because 80% of our taste is made up by smell.