generate static date and serial number when data is entered in any of three cells in row

Here you go. This belongs in the ThisWorkbook object.

If you can come up with a rule of how to get the XXX (GSA, etc) then we can write that into the code. Otherwise they all need to be hardcoded as the two from your example are now.

Private Sub Workbook_SheetChange(ByVal ws As Object, ByVal Target As Range)


If (Target.Column = 4 Or Target.Column = 5 Or Target.Column = 6) And Target.Row > 1 Then
    Application.EnableEvents = False
    If ws.Range("A" & Target.Row) = "" Then
       ws.Range("A" & Target.Row).Value = Format(Now, "yyyy-mm-dd-hh:mm")
    End If
    If ws.Range("B" & Target.Row) = "" Then
        s = ws.Name
        If s = "George's - Cassville" Then
                t = "GCM"
        ElseIf s = "George's - Springdale" Then
                t = "GSA"
        End If
        If ws.Range("B" & Target.Row - 1).Value = "Serial Number" Or ws.Range("B" & Target.Row - 1) = "" Then
            x = "01"
        Else
            x = CLng(Right(ws.Range("B" & Target.Row - 1).Value, 2)) + 1
            If x < 10 Then x = "0" & CStr(x)
        End If
        ws.Range("B" & Target.Row).Value = t & "-" & Format(Date, "yymmdd") & "-" & x
    End If
    Application.EnableEvents = True
End If

End Sub