Calculation of Average in SQL Statement

Hello,
I would appreciate help regarding this:
I have bellow VBA code which I use to select and export data from MS Access database to MS Excell worksheet. It works very fine. But I have problem because I need to set my bellow strSQL statement in way to have also AVERAGE calculated on D_1,D_2 and D_3 values within the same. It would (according to my understanding)look something like this:
strSQL = " SELECT ID, NAME, PP_NO, D_1, D_2, D_3, AVG(D_1,D_2,D_3) from TBL_1 WHERE DATE = " & Me.Combobox.Value& " "

Also I find As huge problem for me(that also must be incorporated in my VBA) is the fact that Average must not take into consideration D_1,D_2 or D_3 if some of them is Null. It means if D_1 = 4, D_2 = 5 and D_3 = 0 then average = (D_1+D_2)/2 because D_3 is Null. Otherwise, if for instance D_3 = 10 then average is (D_1+D_2+D_3)/3
This Average is calculated on level of each ID which are a number of aproximate 100 positions within the table and each one is unique.
Please your advice or (even better) solution how to solve this problem.
Thank you very much.
Kind regards

Private Sub Start_Command_Click()
Dim col As Integer
Dim row As Integer
Set cnn = CreateObject("ADODB.Connection")
Set rst = CreateObject("ADODB.Recordset")
 
stDB = "MY path to database\db2.mdb"
 
cnn.Open "Provider=Microsoft.Jet.OLEDB.4.0;Jet OLEDB:Database Password=;" _
& "Data Source=" & stDB & ";"
 
strSQL = " SELECT ID, NAME, PP_NO, D_1, D_2, D_3 from TBL_1 WHERE DATE = " & Me.Combobox.Value& " "
 
rst.Open strSQL, cnn
 
 col = 0
 
 row = 6
  Do While Not rst.EOF
    row = row + 1
    col = 0
    
    Do While col < rst.Fields.Count
      Cells(row, col + 1) = rst.Fields(col).Value
      col = col + 1
    Loop
  
    rst.MoveNext
  Loop
 
Set rst = Nothing: Set cnn = Nothing
End Sub

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