Sub SumEvery()
Dim lRow As Long
Dim tCol As Variant
Dim r As Long
Dim result1, result2, result3 As Long
tCol = 1 ' Use letter or number to represent the column
lRow = Cells(Rows.Count, tCol).End(xlUp).Row
For r = 2 To lRow
Select Case Cells(r, tCol).Row Mod 3
Case Is = 0
result1 = result1 + Cells(r, tCol)
Case Is = 1
result2 = result2 + Cells(r, tCol)
Case Is = 2
result3 = result3 + Cells(r, tCol)
End Select
Next r
Cells(lRow + 1, tCol).Value = result1 & " - " & result2 & " - " & result3
End Sub