I like them as a set and utterly love Martha! Bess & Nell are lovely but Mittie sounds off to my ears. The only other nickname for Martha I know of is Marty, I know a Martha who goes by Marty. But that doesn't fit with Bess & Nell very well, does it? Hmm. Bess, Nell & Marta, perhaps?
Marta does sound very cool. The only reason why I used Mittie in the question are that Mittie fits the double letter theme and it was TheodoreRoosevelt's mother's nickname.That reminds me . . . His sister was Corinne nn Connie. That one kind of works too.