You can use the central panel for both: common ancestor text in the neutral colour, and differences between the common ancestor and the resolved text in colour, with one colour for differences that come from the 'left' set of changes, and one colour that come from the 'right' set.

This is what Araxis Merge did, or at least used to do when I used it, and I found it more useful than P4Merge's 4-way display (which just takes up more space than necessary).

It's possible there are types of merge where it's useful to have the common ancestor permanently visible, but I never seemed to run into them.