Hi, welcome to the board!!
Let's see, first of all, make sure your variables are correct. LastRow should be Long, not String. It's really unclear as to what you are trying to do in regards to where your data is. An uploaded example workbook would work best. Maybe you can try ...
Sub InsertRows()
Dim x As Long, StartRow As Long
With Sheets("Sheet1")
StartRow = .Range("B1000").End(xlUp).Row
For x = 1001 To StartRow Step -1
Select Case Range("B" & x).Value
Case "a", "b", "c", 1, 2, 3
Range("B" & x).EntireRow.Insert
End Select
Next x
End With
End Sub
This will work with all the Case's specified between the StartRow and LastRow.