You could stop for editing on the third commit
p f70f90f ... some message ...
s 514ee77 ... some message ...
e ab01b05 ... some message ...
p b109cc1 ... some message ...
that way you stop at ab01b05
for editing. So if you
git reset --soft HEAD^
you can then do your changes. Then
git commit --amend
will squash the changes to the last commit (which now is the squash of f70f90f
and 514ee77
)
and you are set.
After that a
git rebase --continue
will take you to the next step of your rebase.