VBA EXCEL OFFSET HELP!

I have the script working perfectly for me, but would like to take it to the next level. This script will check B3 cells and on for a value. If it has a value, it will copy the value in K column and paste it in a batchfile. What I need to figure out is how can the one script, select and copy column K, L, and M instead of just K. This is the script:

Sub Tester()
    Dim LastCell As Range
    Dim ColumnDiff As Long
    Dim newusers As String
 'Setting the Offset
  ColumnDiff = ActiveSheet.Columns("K").Column
  ColumnDiff = ColumnDiff - ActiveSheet.Columns("B").Column
 
  'Select, Copy, Paste
  With ActiveSheet
    Set LastCell = .Range("b3").End(xlDown)
   .Range("B3", LastCell).Offset(0, ColumnDiff).Select
   Selection.Copy
   newusers = Shell("C:\Windows\notepad.exe \\Sharing200\users\newusers.bat", vbNormalFocus)

   SendKeys "^v"
   SendKeys "^s"

I have tried ColumnDiff = ActiveSheet.Columns("K:M").Column, but that wasn't right.