Both emacs and vim can be run under windows, and both have the ability to display 3-way merges quite well.
For emacs, you'd want magit + smerge or ediff.
In vim, you'd want vimdiff + vim-fugitive for a few handy helpers.
I use CLion (or other Jetbrains IDEs, eg PyCharms on the occasions I'm not working in C/C++/Rust). They've got a very nice built-in 3-way merge diff tool. Non-free for some of the IDEs, but if you're working with a language that has a free "community edition" this is viable. Or if you're willing to pay for a very nice IDE.
I’ll get shot down for it, but I love the JetBrains IDE visual merge tool. It’s not free, but this among other things makes it worth it for professional work.
Not free, but Araxis Merge is pretty good.
> 3-way merge diffs under windows
Is there any reason for not using kdiff3?
Yet another option: p4merge (from Perforce) works great.
Meld is pretty neat
Beyond Compare
It really does live up to it's name in my opinion. Been using it for over a decade, originally in Windows but now macOS