Of all the various barrel lengths, the 3" Python is the rarest.
Back in the early 1980's Colt made up a short run of 3" Pythons for a big distributor in California.
These are variously known as "Combat Pythons" or sometimes "California Combat Pythons".
After the run for the distributor, Colt offered their own 3" Python for a short time.
The Colt production version was simply marked as a Python with no special barrel markings.
Colt made these in blue, "possibly" in bright nickel. I "seem" to remember seeing them in stainless steel also, but I could well be wrong about this.
3" Pythons are very rare, and on the few occassions when one does come up for sale, they bring prices ranging from sky-high to insane.
One problem: After Colt stopped production of the 3" Pythons, they sold the unused barrels to Gun Parts Corporation.
Gun Parts sold an unknown number of these barrels on the commercial market, and a GOOD many 3" Pythons are actually guns that were rebarreled with one of these spare barrels.
In the case of a 3" Python it's absolutely a case of "buyer beware".
In your case, you have the first version, made for the distributor, and so marked on the barrel, and probably on the box, so you don't have to worry that this is a fake.
I'd recommend NOT shooting the gun, unless it's already been shot.
This will preserve it's New In the Box collectability value.
I don't have a feel about market value on this rare Python, but since there were few made, and collectors would be crazy for a NIB gun, I'd say it would be between $1500 and $2000 dollars.