Private Declare Function GetKeyState Lib "user32" (ByVal nVirtKey As KeyCodeConstants) As Integer
Public Function MButtonDown(btButton As Byte) As Boolean
Select Case btButton
Case Is = 1
MButtonDown = CBool(GetKeyState(vbKeyLButton) And &H8000)
Case Is = 2
MButtonDown = CBool(GetKeyState(vbKeyRButton) And &H8000)
Case Is = 3
MButtonDown = CBool(GetKeyState(vbKeyMButton) And &H8000)
End Select
End Function

Private Sub Form_Load()
If MButtonDown(1) Then MsgBox "Левая клавиша нажата!"
If MButtonDown(2) Then MsgBox "Правая клавиша нажата!"
If MButtonDown(3) Then MsgBox "Средняя клавиша нажата!"
End Sub