Is it reasobnable to expect 3" groups fired 2 handed at 15 ft from either a .36 or.44 cal revolver using Pyrodex and seabed balls?
I don't know what a "seabed ball" is, but yes, you could easily get 3" groups at a mere 15 feet using a 2-handed grip. Many people achieve 4" groups at 25' with a one-handed grip.
The key is to develop an accurate load shooting off a bench rest.
For .36 caliber, you will probably want to start at 12 grains and work up in 2 grain increments until you find your optimal group. For .44 caliber, start at 15 grains and work up.
You will probably find optimal groupings for .44 caliber at around 18 grains of 3F black powder. I don't know what you will find for substitutes like Pyrodex.
I load my balls with powder, then cream of wheat filler, and then the ball, such that the ball, when firmly seated on the filler, is flush with the face of the cylinder. Then I apply lube over the balls.
When you develop an accurate load off the bench, then you know what the firearm is capable of. The rest is up to you.
Steve