Thirding it except I do it from Emacs. Three side-by-side pane with left / common ancestor / right and then below the merge result. By default it's not like that but then it's Emacs so anything is doable. I hacked some elisp code a great many years ago and I've been using it ever since.

No matter the tool, merges should always be presented like that. It's the only presentation that makes sense.