How do I use Meld as a merge tool with Sourcetree on Windows?

I feel the existing answers slightly missed the point. Here is my own dog food:

Arguments Detail:

Diff:  $LOCAL $REMOTE
Merge: $LOCAL $BASE $REMOTE --auto-merge --output=$MERGED

For External Diff, you need to remove $BASE from your argument list.

For 3-way merging, you need to click on the External Merge Tool option instead, which will only be available if there are any unsolved conflicts.

If you are not restricted to Sourcetree + Meld, I reckon the Git Extensions + KDiff3 suite could be a good open-sourced alternative also.