Your code should be
Dim aDim As String: aDim = "a" Dim bDim As String: bDim = "b" Dim cDim As String: cDim = "c" Dim test As String: test = "c" Select Case test Case aDim MsgBox "It's an a" Case bDim MsgBox "It's a b" Case cDim MsgBox "It's a c" Case Else MsgBox "Something went wrong" End Select

At first glance...
Dim a As String: a = "a" Dim b As String: b = "b" Dim c As String: c = "c" Dim test As String: test = "c"
Select Case test Case a MsgBox "It's an a" Case b MsgBox "It's a b" Case c MsgBox "It's a c" Case Else MsgBox "Something went wrong" End Select