I would first look and see what is available in your local area.
You can go a long ways in USPSA and IDPA with a single gun, belt holster, some magazines and mag pouches. Two of the local clubs here run outlaw 3 gun on the side and that seems common else where from the people I have talked with.
If you do want to try 3 gun without buying a shotgun just show up for a match with a bulk pack of 12 gauge ammo and someone will let you use their gun to help make a new shooter feel welcome.
http://www.uspsa.org/locate-uspsa-clubs.php
http://www.idpa.com/clubs.asp