I purchased Araxis Merge but I still find myself using Meld for the vast, vast majority of my 2- and 3- way Diffing.

It’s slightly tricky to integrate into Sourcetree but once you configure Sourcetree with the correct command line args it works quite painlessly.

Meld is extremely performant too.

Another text editor I love is “010 Editor”, it’s the only windows app I’ve found that works well on 2+ GB files without slowing my computer down.