So, did you remove operations on the first two files and attempt the modifications on the third one by itself, to rule out that it's not the addition of the third file that breaks things, but it's ONLY the third file that's broken?

Do so. This is called "halving the problem". If it's only the 3rd file that's borked, you can focus on it alone.

Also, that's a lot of unneeded repetition, which is always prone to mistakes. Use a loop as suggested.