Auto Lock Worksheet except if reference cell is blank

I am working on a Log sheet where in the team will need to record all guest's requests. The worksheet will be automatically locked 3 days after todays date has passed. I already have a VBA however it auto locks the worksheet since the reference cell is blank. May I kindly ask for your assistance on how the macros will not work if the cell reference is blank. Please see below VBA used;

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    If Sheets("Request").Range("B3").Value < Date And Sheets("Request").Range("B3").Value < Date - 3 Then
        MsgBox "This workbook is LOCKED for editing."
    End If
End Sub

Thank you for your help

Hi mikeTron,

Thank you so much for your help. At first it didnt work with Sub check and I replaced it with Selection Change and it worked perfectly.

Greatly appreciate your help