RDLC grouping nested with iiF construction

I have a report with a table that is sorted and grouped in three different ways.
I managed to get 2 ways of sorting in one table in de RDLC layout like this:
Sorting/grouping:
iiF(Fields!SortInt.Value = 1, Fields!MIL_ItemCategoryCode.Value,Fields!MIL_No.Value)
(and de table is shown for Sortint=1 and sortint=2)

Now I want the 3th sorting/grouping also to be nested...but can anyone tell me how to write a grouping construction like shown in the image ?
<Group Name="MILFor1_Grades">
<GroupExpressions>
<GroupExpression>=Fields!MIL_No.Value</GroupExpression>
<GroupExpression>=Fields!MIL_GradeCode1.Value</GroupExpression>
<GroupExpression>=Fields!MIL_GradeCode2.Value</GroupExpression>
<GroupExpression>=Fields!MIL_GradeCode3.Value</GroupExpression>
</GroupExpressions>
</Group>

Now it is a different table in de body….

Thanks

Carolien