looking for a merge tool.

I'm doing a bit of merging fromthe NetBSD USB code to -current,
but I'm wonderring if anyone can point me to a good merge tool.

I'm currently using mgdiff which shows the two files in
adjoing panes with the differences coloured and allows you to
select the diffs from the left or right windows. quite good until
you need the third option, which is "both" or "the left, but with
a small edit"

does anyone know of a tool that gives 3 panes and allows manual
editing of the merged product?

Julian