Custom Field

I’ve been looking for a solution for ages.
There is this wordpress page: http://www.tammyhartdesigns.com/work/
I’d like to create something like that one. I’ve already created a template page similar to that where only excerpts of posts of certain category are displayed. I mean a loop. My only tiny problem is to put three thumbnails before the excerpts of each post such as you can see the site above. I’ve e-mailed the author of the site and she told me : “That page is using three custom fields to store image url’s associated with blog posts in the “portfolio” category.”
Ok any ideas how to do that?