You won't be able to learn too much in 3 days, and also none of the guys at a startup will take a lot of interest making you understand things considering the short tenure.
Pick up a small idea - some pain point that you see in your daily workflow. There are so many possibilities with free infrastructure today, Heroku, Google App Engine, and of course Amazon WebServices.
Start contacting people to see if they are interested and go through the proper way how someone seeds a startup. This will help you learn something that no experience ever will.