What an interesting dilemma! Here is my suggestion and it just might turn out to be fun for all. If you and your husband really want a middle name for your third child, then I think the first two also need a middle name, albeit a bit tardy. They are young enough that it would be fairly simple, I would imagine, to actually have their middle name inserted as a legal name- change on their birth certificates and social security cards. (Note: It is my understanding that it does not have to be actually documented and that one can use any name one wants, so long as it is not for illegal purposes...however, at the six-year-olds' very young age, I think I would go on with the legal route, which likely would only entail filling out a few forms and paying a modest fee. You might "Google" "changing name officially" to get the particulars on this).
O.K., back to the the middle name for the twins. What fun it would be for each of them to choose his own middle name!
If they want to do this later (add a middle name), it will become somewhat more complicated once they change schools, enter college, enter the workforce, etc., as they will have used the non-middle name monikers for so long, various records will perhaps conflict with their suddenly appearing at work, say, with a middle name at age 25. So, I would go on and let them each choose a middle name now, to coincide, more or less, with their new little brother's arrival. Good luck with all three!