how can I skip the thumbnail images, and go straight to the Fullscreen mode of the Image viewer included within the Three20 Framework.
One solution (my own experience): I followed this Three20 tutorial and set up a full screen image viewer controller. Then I simply presented the controller modally. It shows the full screen viewer perfectly. However, the user would be able to access the thumbnail view if they click the "see all" button on the nav bar(right button), which, as for my app, isn't a bad idea. If what you want is get rid of such a thumbnail view completely, take a look at three20's source code and override the part that generates the thumbnail view and you'll be fine.
An alternative: Kick Three20 out of your project and try this simple yet powerful stand-alone full screen image viewer from enormego.