DeSantis makes a thumbreak belt holster for the 3".
You could go with a belt slide, like the Bianchi model 5.
El Paso Saddlery has a bunch of holsters they could make for that gun.
I built this one for my Smith M60 with a 3" tube several months ago, Drag. It's a Tom Threepersons type wiht open top, fitted snug so no keeper strap is necessary. El Pason Saddlery makes a similar model, and the quality is superb.
I've tried a shoulder rig, but found that it flopped around too much and the draw was slow. With a shirt tail out or a jacket, this one works well. The 2nd one shows the same type of holster, albeit without the reinforcing band, with my wife's M36 in place. HTH's Rod