VBA code to sum values on two different columns and result in another column

Try this on for size:

Sub total()
    Dim MyRange As Range
    Dim i As Long
    Dim lr As Long
    lr = Range("A" & Rows.Count).End(xlUp).Row

For i = 2 To lr

    With Worksheets("Sheet1")
      Set MyRange = .Range(.Cells(i, 1), .Cells(i, 3))
      .Cells(i, 4) = WorksheetFunction.Sum(MyRange)
    End With
Next i
    

End Sub