SSRS - add 3 rows based on first letter of last name

Hello,

I'm working on a report that has details associated with first and last names ordered by LastName.

I'd like the report to add/display 3 empty rows between groups of last names(by their first letter).
So groups of "A" LastNames would end with 3 blank rows. Groups of "B" LastNames would end with 3 blank rows and so on all the way through "Z"

Example Report:
Made up LastName column:
Abbe
Adams
Arlyn
New Row
New Row
New Row
Bird
Bryn
Burn
Bzork
New Row
New Row
New Row
Cnames begin and so on. . .

Does anyone have a best practice or an expression they could share to do this?
I've come close with using a hidden expression =IIF(Left(Fields!LastName.Value,1)>(Previous(Left(Fields!LastName.Value.1))),False,True), but I almost need a "Next" instead of a "Previous" to get that to work correctly, but I don't think there is a Next function available.

Any advice would be greatly appreciated. Thanks!