You were on the right track. Say you have data like this:
I have created a tablix like this:
The Row Group expression is:
=Ceiling(Fields!Row_Count.Value / 3)
This works together with the Column Group expression to split over three columns:
=(Fields!Row_Count.Value - 1) Mod 3
The other thing to note compared to your tablix is that CustomerName is not in a table header row, but rather there are two row header rows, one for CustomerName and one for Details.
This is looking OK to me, obviously you can format to taste: