To be honest since your new the shotgun reloading is going to be awkward and something you will need to work on. In 3 gun there are two major skills to practice and are the major factor of competency. That is shotgun reloading, and long range rifle shooting. Most guys when they come out use whatever they have including the shotshell bags common for sporting clays, trap and skeet. Your bandoleer will work. Use whatever you got there is no shame. We have seen every type of thing possible from new guys loading out of their pants pocket or cargo pocket to plastic shopping bags hanging off the waist. All anyone will ask is you be safe, particularly the 180 and where you point your guns. As an IDPA guy you probably have an handle on this and will be just fine. Just show up as normal for sign up and tell them you are new. They will probably have a resident "show the new guys the ropes" squad they will make sure your safe and have a good time. One great thing about competition shooting is everyone loves the new guys. That is until you start challenging the guys that put in their time. Then things get really fun.