Excel Script - Find And Replace Data

I am looking to make an excel script that can find and replace data.

Situation:
A--------B-----------C
Cat------Dog------Banana
Dog------Fish------Apple
Fish------Cat-------Orange

So the macro would look at the data in a cell in column B, then look at the adjacent cell in column C, and replace all instances of that data in column A with what if found in C. So the results would be:

A------------B-----------C
Orange------Dog------Banana
Banana------Fish------Apple
Apple--------Cat-------Orange

But that's not all, I would like it to not change cells in A that already have been changed once! (I'm trying this with changing the background colour)

I am at a complete loss and need to find and replace over 3000 items.