All the different games have rules, here are some links to some of the different rulebooks.

Do yourself a favor and take a look at the rules before you show up for a match.

IPSC/USPSA. http://www.uspsa.org/rules/2008HandgunRulesindexed.pdf

IDPA. http://idpa.com/Documents/IDPARuleBook2005.pdf

3-Gun. http://coloradomultigun.com/localrules.html