Code:
'general declarations
Dim BioPhysChem(2) As Boolean

Private Sub cmdBiology_Click()
BioPhysChem(0) = True
BioPhysChem(1) = False
BioPhysChem(2) = False
End Sub

Private Sub cmdPhysics_Click()
BioPhysChem(1) = True
BioPhysChem(2) = False
BioPhysChem(0) = False
End Sub

Private Sub cmdChemistry_Click()
BioPhysChem(2) = True
BioPhysChem(0) = False
BioPhysChem(1) = False
End Sub

Private Sub cmdDiagram_Click()
Select Case True
Case BioPhysChem(0)
' biology code
Case BioPhysChem(1)
'physics code
Case BioPhysChem(2)
' chemistry code
End Select
End Sub
this is more work then necesary, you COULD use an option button,
it would be simpler, and there would be less code, but this works too.