The difference in the ways that farmers artisans and scribes contributed to the civilization of ancient egypt is that

Farmers were the ones who provided food for the civilization of ancient Egypt, they were the backbone without which it couldn't exist. 

Artisns were the group of people who provided Egypt with the tools that were needed to function properly. And not only food, clothes and other similar necessities were also provided by them.

Scribes were a class of people who were writing down the history of ancient Egypt and are also the main reason we know so much about this civilization.