From what I have found a 3 gun tactical match is basically a shooting match where you use a pistal ,shotgun and a rifle. Beyond that I dont know but maybe this link will help you out.
http://vikingtactics.com/3Gun/5th/MatchRules.htm
This tells a little more about it with pictures
http://www.rbgc.org/actionrifle/ARpage.html
http://vikingtactics.com/3Gun/5th/MatchRules.htm
This tells a little more about it with pictures
http://www.rbgc.org/actionrifle/ARpage.html