What's a diff? See WP:DIFF. Your user page should look something like this for your second assignment. Three diffs are listed below under a level 5 subheading.
I'm editing Wikipedia as part of this assignment and here's a link to my sandbox.
These are random examples. If you made three edits to an article but you want to show it as one diff, you can do that. If you're curious about the explanatory footnote I added in the third diff, see the short article prothrombin G20210A as an example. That article is also good with demonstrating the correct placement of references/inline citations (WP:REFSPACE).
