I've used that exact gun for quite a bit, although no 2 are the same I hope this can aide you.
3" #4-BBB's @1450fps or less, depending on what is flying. With a good dense pattern you won't need hevi-shot.
The 835 has basically a 10 gauge muzzle dia. So if you choke it modified it will throw a modified pattern when using steel.
I would recommend the mossberg, improved cylinder, or, the modified, ulti-mag choke, and if you got a couple extra bucks laying around Patternmaster "over decoy" choke tube from
www.patternmaster.com, they make a great choke. Unless all your shots are 40+ yds, the more open choke will take away a lot of frustration.
A good rule of thumb: The faster the load, the heavier the shot weight, the less dense the pattern. The larger the shot, the tighter the pattern.
I would not bother with the 3.5"s, or any load with more than 1-1/4oz of shot{ just because there are more pellets in the shell doesn't mean there will be more at the target}. I would stick with 1-1/8oz - 1-1/4oz 3" shells at 1450fps or less. Geese will fall from BB's & BBB's, mallards will fall to 3's - 2's.
I like Remington and Federal Loads, the pellets are actually round, compared to winchester which are not.
If you get serious enough compare some 3.5" shells to some equal velocity 3" shells on paper and youl'll see what I mean.
I've never tried but have heard from a lot of people that Kent Fast Steel products are great. Cabellas caries them.
Have a good one.