Merge three Datatables deleting duplicates rows after copying their column data to the remaining row, finally sorting twice

Question

I hope someone can help with this, as I believe I have made it overly complicated...

This is the structure of the datatables

With FirstDT
    .Columns.Add("Code")
    .Columns.Add("Description")
    .Columns.Add("Expiration Date")
    .Columns.Add("TableIdentifier")
    .Columns.Add("Quantity")
End With

With SecDT
    .Columns.Add("Code")
    .Columns.Add("Description")
    .Columns.Add("Expiration Date")
    .Columns.Add("TableIdentifier")
    .Columns.Add("Quantity")
End With

With ThirdDT
    .Columns.Add("Code")
    .Columns.Add("Description")
    .Columns.Add("Expiration Date")
    .Columns.Add("TableIdentifier")
    .Columns.Add("Quantity")
End With

With SumDT
    .Columns.Add("Code")
    .Columns.Add("Description")
    .Columns.Add("Expiration date")
    .Columns.Add("Quantity of first")
    .Columns.Add("Quantity of second")
    .Columns.Add("Quantity of third")
End With

I want to merge the first two datatables and then join the resulting datatable with the third one where columns 'code' and 'expiration date' match. After which I want to find the rows which have matching 'code' and 'expiration date' columns, and then import them to SumDt. Then I need to locate duplicate rows as per 'code' and 'expiration date', add the 'quantity' column of the duplicates to the corresponding column of 'SumDT' datatalbe and delete the duplicates.

Here's what I got:

SumDT.Merge(FirstDT, True)
SumDT.Merge(SecDT, True)

SumDT.DefaultView.Sort = "Code ASC"
SumDT= SumDT.DefaultView.ToTable

Dim foundrow() As DataRow
For i As Integer = 0 To SumDT.Rows.Count - 1
    If i = 0 OrElse SumDT.Rows(i).Item(0) <> SumDT.Rows(i - 1).Item(0) Then
        foundrow = ThirdDT.Select("Κωδικός=" & "'" & SumDT.Rows(i).Item(0) & "'")
    Else
        foundrow = Nothing
    End If

    If foundrow IsNot Nothing Then
        For Each row As DataRow In foundrow
            SumDT.ImportRow(row)
        Next
    End If
Next


Dim FinalDic As New Dictionary(Of String, String)

For i As Integer = 0 To SumDT.Rows.Count - 1

    key = SumDT.Rows(i).Item(0) & "#" & SumDT.Rows(i).Item(1) & "#" & SumDT.Rows(i).Item(2)
'Item(6) is the tableIdentifier and item(7) is the quantity
    value = SumDT.Rows(i).Item(6) & "#" & SumDT.Rows(i).Item(7)

    If Not FinalDic.ContainsKey(key) Then
        FinalDic.Add(key, value)
    Else
        FinalDic(key) = FinalDic(key) & "#" & SumDT.Rows(i).Item(6) & "#" & SumDT.Rows(i).Item(7)
    End If
Next

With SumDT.Columns
    .RemoveAt(7)
    .RemoveAt(6)
End With

SumDT.Clear()

Dim numofvalues As Integer = 0
For Each pair In FinalDic
    numofvalues = Split(pair.Value, "#").Length
    SumDT.Rows.Add(Split(pair.Key, "#")(0), Split(pair.Key, "#")(1), Split(pair.Key, "#")(2))
    For i As Integer = 0 To numofvalues - 1 Step 2
        If Split(pair.Value, "#")(i) = "008" Then
            SumDT.Rows(SumDT.Rows.Count - 1).Item(4) = Split(pair.Value, "#")(i + 1)
        ElseIf Split(pair.Value, "#")(i) = "098" Then
            SumDT.Rows(SumDT.Rows.Count - 1).Item(5) = Split(pair.Value, "#")(i + 1)
        Else
            SumDT.Rows(SumDT.Rows.Count - 1).Item(3) = Split(pair.Value, "#")(i + 1)
        End If
    Next
Next


SumDT= SumDT.AsEnumerable().OrderBy(Function(r) r.Field(Of String)("Code")).ThenBy(Function(r) r.Field(Of String)("Expiration Date")).CopyToDataTable()

DataGridView1.DataSource = physStockDT

Finally I need to sort the datatable by code and then by date, problem is because not all the values of column 'expiration date' contain actual dates but also empty strings/nothing, it won't let me do r.Field(Of Date). Even after converting the column to Date datatype and set AllowDBNull to true for the column, it always throws error saying it cannot cast it.

While this works, I know it's messy, so I'd appreciate any advice to help simplify the code and make sorting by date work as intended.