I shoot two or three per year. I don't have the gear to be very competitive, but I do alright with what I have. I look at them as an opportunity to get trigger time with guns that I may have to count on some day, but that otherwise wouldn't get much use. We have a "big" 3-gun match in central Washington state, every Spring, and a group of four or five of us from the Seattle area would go each year. We started doing shotgun and rifle side stages at our monthly IDPA matches in '04, and the interest in 3-gun has gone way up. I think a dozen of us are making the drive in April. Even though IDPA doesn't have rules for 3-gun, some clubs are writing their own, and we may host a 3-gun match this Summer, and see how it goes.
Typically, guys are shooting whatever pistol they use in IDPA or IPSC matches, an AR, and a Remington shotgun (1100, 11-87, or 870). Your gear doesn't have to be fancy, but it has to run reliably. Be able to hit with your rifle, out to 300 yards or more, and practice stoking-up your scattergun. I shoot a M1 Carbine, which is pretty dicey at 350 yards, but is great on house-clearing and jungle-run style stages. Likewise, a pump shotgun isn't great for clearing a plate rack, but it's the reloads that make all the difference, as you will often have stages with five seconds of shooting and fifteen seconds of reloading.
Do you have specific questions?