> I must say their 3-way merge tool is the best free software on Windows the only competing one, but less good, is p4merge, and it's closed source.

A long time ago, I used Araxis Merge[1] and I can strongly recommend it[2]. It was specifically better than both tortoise git and p4merge, after having used both of those options personally.

[1] https://www.araxis.com/merge/

[2] Assuming you're stuck in a windows development environment - there might be better tools available if you're not on Windows.