Create validation list of distinct values

Hello everybody
I have three columns A - B - C with values ..
I want in Range("E1") to create validation list of distinct values of the three columns using vba ??

I tried this code

Sub Unique()
    Dim r As Range, cel As Range, cell As Range
    Dim d, a
    Dim LR As Long, LastRow As Long, X As Long, Y As Long
    LR = Range("A" & Rows.Count).End(xlUp).Row
    Set r = Range("A1:C" & LR)
    
    On Error Resume Next
    
    Set d = CreateObject("Scripting.Dictionary")
    For Each cel In r
        If cel <> 0 Then d.Add CStr(cel), CStr(cel)
    Next
    a = d.items
        
    With Range("E1").Validation
        .Delete
        .Add Type:=xlValidateList, Formula1:=Application.Transpose(a)
    End With
End Sub

But It doesn't work for me .. I don't know why?

Thank you very much Mr. Debraj Roy
You are very helpful