Sum last 3 columns in Excel using macros - vba

Question

In an Excel File, I need to find out the last 3 columns since because my column length gets updated each month when new data comes and need to do a sum on that last 3 columns values as I want recent data and put the sum in the new column.
EG:
A B C D
1 2 3 4
5 6 7 8
1 2 3 2
In the existing sheet, I want to find B, C, D as recent 3 columns (last 3 columns) and sum it up and put in new column E.
A B C D E
1 2 3 4 9
5 6 7 8 21
1 2 3 2 7
Please help me with ways to automate it with Excel macros. Thanks in advance for your help. I created a code but it's not working
Dim LastCol As Integer
With ActiveSheet
LastCol = .Cells(1, .Columns.Count).End(xlToLeft).column
End With
MsgBox LastCol
Dim LastRow As Integer
With ActiveSheet
LastRow = .Cells(.Rows.Count, "H").End(xlUp).Row
End With
MsgBox LastRow
Dim j As Integer
For i = 2 To LastRow
For j = LastCol - 2 To LastCol
Range(i & LastCol + 1).Value = Range(i & j).Value + Range(i & j + 1).Value + Range(i & j + 2).Value
Next j
Next i

Answer 1

Put this formula in E1
=SUM(OFFSET(E1,0,-3,1,3))
to get the sum of the 3 columns before column E.
Any time you insert new columns before E it will adjust automatically to the last 3 columns right before the sum column.
For more information have a look at OFFSET function.

Answer 2

Use Arrays , Whenever you have to work with multiple cells,rows or columns. Use Arrays. First you hold the value in array and work with the array in memory.
Try to minimize the interaction with the Workbook/Worksheet/sheets.
It will make you code less slow and less error prone.
Here is the code for your reference :
Function getsum()
Dim wb As Workbook
Dim ws As Worksheet
Dim tot_row As Long
Dim row_to_start As Long
Dim cl As Long
Dim irow As Long
Dim sumarr As Variant ' Declare the array
Set wb = ThisWorkbook
Set ws = wb.Sheets(1)
sumarr = ws.Range("A1").CurrentRegion ' Put the current region in the array
tot_row = UBound(sumarr, 2) 'Get total rows
row_to_start = tot_row - 2 ' Get the rows from where sum process with start
For cl = row_to_start To UBound(sumarr, 2) ' start from 3rd last column till last column
For irow = LBound(sumarr) + 1 To UBound(sumarr) ' from second row to the last row
msum = msum + sumarr(irow, cl) ' do the sum
Next irow
Next cl
getsum = msum ' return the sum
End Function

Answer 3

Thanks a lot for your precious time to reply.
I have found a solution in Excel VBA macro and it works very fine!!
Dim start As Integer
Dim Mid As Integer
Dim finish As Integer
start = LastCol - 2
Mid = LastCol - 1
finish = LastCol
For i = 2 To LastRow
For j = start To LastCol
If Not IsEmpty(Range("D" & i).Value) Then
Cells(i, LastCol + 1).Formula = "=SUM(" & Cells(i, start).Address & ":" & Cells(i, finish).Address & ")"
End If
Next j
Next i