Do you have an example that has been built already?
My thinking would be to add a number to a variable for each button clicked - when the variable gets to 3 you would know that all three objects had been selected. Logic would be something like the below:
Dim bClicked As Integer
If Button1clicked Then
bClicked = bClicked + 1
End If
If Button2clicked Then
bClicked = bClicked + 1
End If
If Button3clicked Then
bClicked = bClicked + 1
End If
If Button_clicked > 2 Then
' do something
Button_clicked = 0
End If