Why does vimdiff open with three windows when merging branches?

Question

vim

Merging branches opens vimdiff with three windows. The purpose of each?
At random, it was found that the first window is the branch into which the changes are poured.
The second window - which branch we merge with the current one. Why do you need a third - in the fog.
The task of setting up vimdiff seems to be handled by mercurial, but the docs don't say anything about it.

Answer 1

This should be the standard view for a 3-way merge -- a common ancestor, changes in one branch, and changes in a second branch.

Answer 2

for sure hg has options for aliases or diif-tools settings (there are definitely some in git) you can specify only two files instead of opening three