No major or minor. 9mm is the most desired caliber for handgun in 3 gun. Unless you plan to run in heavy division.
Every match is run by its own set of rules and basically is its own entity. It can be a tad bid confusing to try and grasp initially. Best advice is just go shoot and don't worry about the rest. Know the rules of the match you are shooting, be safe and have fun. The rest will slowly fall into place.