My IDPA club is running the classifier this weekend.

The side match to keep people occupied is a 3-gun match.

I'll be shooting my Winchester 120 shotgun, a Ruger 10/22 and my govt-size Colt Series 70.

The 10/22 is the only suitable "rifle" I currently own and I'll be shooting the classifier for CDP so I'll just stay with the 1911.

I'm not sure how extensive the 3-gun course will be.

The first thing I see is the 10-22. If this is a fun match you may be ok but normally you will need at least a 223 to shoot the rifle stages. I would look around and see if another member would loan you one for the match. Make sure you have a way to carry extra shotgun ammo and spare mags for the rifle and pistol.
the IDPA stages are small compared to 3-gun stages so having enough mags to shoot 40 rounds from each gun is common. Practice loading the shotgun this is the one thing that beats most shooters. Don't be afraid to sinle load it either, once you practice it can be done fairly fast. The other thing that bites shooters is not keeping the finger off the trigger during moving and reloads. All I can sasy is PRACTICE, PRACTICE PRACTICE.

Oh yeah HAVE FUN

3-gun can be as addictive as Kraack..... There goes the retirement nestegg, LOL