In order to change the search result page from 3 to 1 columns, you need to edit the file called “catalogsearch.xml” which is located in the folder “app/design/frontend/[your_theme]/default/layout/”.
In that file, you need to find the line that says:
and change “3” to “1”. So it will look like this:
Save the file and flush the Magento cache (System » Cache Management) and you should be good to go!