One limitation of Paul's approach is that the macro would get also get called
if the cell were entered via the ARROW keys or the TAB key or even the enter
key.
Another approach is to Insert a hyperlink in B3 to a Place in the Document
(namely B3). At first blush, this seems a little silly (why insert a
hyperlink that goes nowhere?). We now can trap the click with a hyperlink
follow event macro:
Private Sub Worksheet_FollowHyperlink(ByVal Target As Hyperlink)
If Target.Parent.Address = "$B$3" Then
Call hello
End If
End Sub
Now the cell will perform just like a button. You can even add a mouse-over
popup message if you want.