Is Matilda always called Matilda or is she Matty?
If she is Matilda then I'd go f8r the third name ending with an a or er
Oliver
Peter
Rebecca
Christina
If Matilda is Matty I would more go for something else that could be shortened to two syllables ending in eee
Frankie is what popped into my head after imagining yelling upstairs for the kids to get a move on "Noah! Matty! Frankie!"