I would like to request support for 3-way merge using external diff tool instead of the default auto-merge. The feature can be seen for example in docs] or web] and was discussed the mailing list].

Bazaar creates 3 files in addition to the auto-merged file, which can be passed to the external merge tool. A bzr resolve command is then called which checks the merged file and deletes the temporary files.
Perforce uses internal merge tool which accesses the repository directly.
Since the Perforce-way would require support for accessing fossil repos directly, a reasonable way would be using the temporary files as bazaar does.

Many tools are capable of this type of merge (meld, diffuse, kdiff, winmerge, ...)