"OK, lets do this!"
The secret shirt in Dangeresque 3 is the Bearalope from Graham Annable's Dank webcomic on the Telltale website. To get the shirt, jump off of the ledge in the Snap Shak until Strong Bad lands upside down (you can't be wearing a costume for this to work). Now try to enter the door from the bottom. If all goes well, Strong Bad should now be wearing the shirt.