What are the differences between double-dot “..” and triple-dot “…” in Git diff commit ranges? [duplicate]

What are the differences between the following commands?:

git diff foo master   # a 
git diff foo..master  # b
git diff foo...master # c

The diff manual talks about it:

Comparing branches

$ git diff topic master    <1>
$ git diff topic..master   <2>
$ git diff topic...master  <3>

1. Changes between the tips of the topic and the master branches.
2. Same as above.
3. Changes that occurred on the master branch since when the topic branch was started off it.

but isn’t totally clear to me.