Did you try adding three banners to the Banners All sidebox?
Go to Tools ... Banner Manager ... and click INSERT ...
Fill in the information for:
Banner Status: YES
Banner New Window No (use Yes if you want to have the banner lead to another site)
Banner on SSL YES (use NO if the image cannot or should not display on secure pages)
Banner Title: My Banner 1 (or some title for banner 1 of 3 banners)
Banner URL: google.com (or an internal link for example: /yourcatalog/index.php?main_page=shippinginfo to have a click on the banner go to the Shipping Info page)
Banner Group: BannersAll (or something for the 3 banners and remember the name you use, NOTE: once the name exists, you can select it in the future from the dropdown)
Image: (browse for the image to use for the banner)
Image Target (Save To): banners/ (to have your image go into the directory /images/banners)
And repeat this for your three banner ...
Next, go to the Configuration ... Layout Settings ...
Find the setting for:
Banner Display Group - Side Box banner_box_all
Enter the Banner Group Name that you used, such as: BannersAll
and update the setting ...
Now go to the Tools ... Layout Boxes Controller ... and turn on the sidebox for Banners ALL ...
Check your site and refresh, and you should now see your 3 banners in 1 sidebox ...