how do I export from Aperture 3 to shutterfly on the web

As there is no plugin for Aperture to export to Shutterfly (as there is for iPhoto) you have two options.

One is to upload your previews from Aperture. When you go to Shutterfly and click the upload link you will have access to the media browser which will allow you to access your Aperture library and upload image previews. Depending on the ultimate use of the images and how you have your preview generation set up this would give you good results.

If you want to upload full resolution images then you will need to export the images from Aperture and them upload those exported images.

Remember if you drag an image oout of Aperture you are getting the preview, you're not exporting a full resolution image.