I have been a participant of 3 gun matches for 3.5 yrs:
1. Guns and gear. It all depends on the club/affiliations/rules etc.
If you are participating at a local club, most likely you will be able to dress as u wish. Most guys have tactical gear that wants to try out so they wear that. Some dress normal. It up to u. If the club is affiliated with associations (idpa.uspsa.etc) they might some rules about it.
Same trend. If affiliated I will not know about rules on what can u put or not on your guns. I used to participated with a bunch of local guys in Las Vegas. Very relax atmosphere.
2. Stages. It all depends on the main guys or 3 gun match head honcho. There is times where u can use 2 guns @ the time. Example shotgun, pistol/ rifle pistol. Sometimes is just single stages. One bay for pistol, one for shotgun, one for rifle.
It is fun fun fun. I recommend 3 gun matches to anybody. U get to learn about the gear u bought or how necessary are those accesories on that rifle. Go and enjoy them as much as u can. Hope this help.