Helena and Matilda sound really lovely together - and fit together well - I think.
I really, really love Clemence, too. I think it's a gorgeous, almost boyish name, but it doesn't seem to necessarily fit in with Matilda or Helena--or both.
You might also like to consider Mathilde (it can be said ma-TEELD or just like Mathilda--Mathilde Jane? Nice!) since it fits well with Clemence because of the... "Frenchness" of them both. (: