If one of them becomes corrupt in a non-obvious way, how do you know which it is? You need three copies to be sure.

Or you could save a hash as well

What if a bit flips in your hash file and changes that? Now you need to store your hash a bunch of times too!

I kid, this is what I do. I've recently been backing up my family photos/videos to various media and I've been hashing each file and storing a hash file alongside the media to compare to in the future.