I'm not sure about definition of "the best way" here, but you can get last 3 rows from DataTable using LINQ like this :
For Each s In ltable.AsEnumerable().Skip(ltable.Rows.Count() - 3)
's is a DataRow here
Next
or without LINQ :
Dim count = ltable.Rows.Count()
For i As Integer = count - 4 To count - 1
Dim s = ltable.Rows(i)
Next i