After Login in your blog and find the follow -> Go to the part whre say "Plantilla / Template" -> After click in the top, rigth part "Crear copia de Seguridad / Restablecer". Choise "Escoger una plantilla desde tu equipo". Select the file downloaded and Done!. Automatically your template is a Denim now with 3 columns.
If you get a trouble leave a message. Thanks!!
More information on this web page:http://ayudaparaelblog.blogspot.com/2007/07/plantillas-con-una-segunda-sidebar-o-3.html