How to get magit ediff to display side by side diffs, and why 3 windows?

When I do a diff with magit, ediff displays 3 windows rather than 2, ie the last commit and the current one, and a third one whose purpose I am not sure of.

Why the third one and how can it be configured just to compare the current file and the last committed just like regular git diff?

How can it also be configured to display side-by-side diffs rather than the vertical ones?