3 hours is not a long time. Pregenerated characters are a good idea - especially ones that contain an easy to grasp 'hook' or are of an identifiable archetype.
Your players may not know the system - introduce the rules gradually during the session, and don't expect all your players to be on the ball with the intricacies of the system. You can always give them a helping hand.
Simple is good - many Kapcon games feature a simplified version of a game, both in terms of setting and system. This allows players to get a 'feel' for the game, and may entice them into finding out more after Kapcon.
It's all about enjoyment - the players are there to enjoy themselves, and enjoy your game. There are many wonderfully supportive players at Kapcon, and the large numbers mean there are people with widely different tastes. If your game is well prepared, well thought out, and you've let prospective players know what kind of game you're running (by writing a blurb and telling the Kapcon organisers what kind of game it is), then you should get a good mix of players, and a good match with your game.
There are fabulous prizes - Kapcon is about good gaming, and good fun. In addition to the experience of running a great game, there is the bonus of being able to win prizes. They aren't the reason people come to Kapcon, they're just a way of acknowledging the hard work and talent of people who attend Kapcon, and make it fun for everyone.