I can't remember everything I've read on the SX3 line...
I think they're lighter than the SX2 due to an alloy mag tube and recoil spring tube. They have a different recoil pad that might be lighter. Looks like the main thing is that they're a half-pound or pound lighter model for model. My SX2 Waterfowl weighs about 8 pounds and is a very soft shooter with 1.5-ounce 3" HeviShot loads. I don't need 3.5" loads and haven't tried any.
I just checked the Winchester site and now I'm confused. The description on the Composite says 6.5 pounds...
"At around 6½ lbs., the SX3 Composite is one of the lightest autoloaders on the market. A weight-saving alloy magazine tube and recoil spring system, slim barrel with a machined rib, and an alloy receiver contribute to the low weight."
The table directly underneath the blurb lists all 4 models, 3" and 3.5" in 2 barrel lengths, at 7 pounds 8 ounces.
I really like my SX2 Waterfowl - it's just right for sitting in a boat and not bad at all for sloshing around a bit in the marsh. The SX3 Waterfowl is listed at 7.5 pounds. Dang, look at the list prices on those babies - $1000 more or less.
John