The first question to ask is whether you really need the commits C4 and C5 to continue your work on C3. If you don't require them immediately, then don't worry about it, continue your work, and merge them later.
If you require C4 and C5 and you have work you don't want to commit yet for C3, then you can do:
git stash
your work in progressgit fetch B
to fetch local copies of branches from Bgit merge B/C5
to get commits C4 and C5 (resolving any conflicts if necessary)git stash pop
to restore your work in progress (again resolving any conflicts)