Your modified choke should be fine......
.....as far as "NEVER shoot steel shot in a full choke": That used to be the advice give, esp. with fixed full choke guns. Newer guns with screw in chokes marked for steel are safe to use. I bring this up because I shoot an 870 with choke tubes, and my "full" tube is marked " for lead or steel".
3" inch shotshells should be fine; I still use 2 3/4 inch handloads. The thing you want to look for is speed....faster is generally better than a bigger payload with steel. In a 3" hull, look for something with a 1 1/8 oz payload and as fast (I like 1500 fps) as you can find!
Shot size? Do you know what kind of ducks you will be hunting? If bigger ducks (mallards, pintails, etc), go with larger pellets, such as #2s...if smaller ducks (teal, wood ducks), you may want to try #4 steel. I use #3 steel in my handloads as a compromise, but I rarely see it in factory shells.
Brand? I would suggest staying away from the "cheap" promo stuff....I've cut those shells open and it looked to me like they swept the rejects off the floor and stuffed them in the shells!!! I would stick with the "higher end" stuff from the ammo makers.....before I started handloading I used Winchester Supremes and they worked very well for me.
I know you said you weren't worried about clothing, but I will offer this piece of advice: Get yourself a good camo headnet....it can mean the difference between bagging birds and having them flare when they see your face shining in the blind.
Good luck, good hunting, and keep us posted on your success....
