Are you talking about the new run of 856's or the old discontinued 856's? The new 856's Have only been out a few months and other than ultra light , and standard weight models there aren't any other versions to select from. The older 856's have a few different barrel length's, but would be very rare to find a 3", and I have not seen any 856's that are ported. Hopefully some of the other more seasoned members will answer.
Taurus parts are difficult to impossible to get, and there are no aftermarket barrels that I'm aware of. Why would you buy this gun if you didn't like the existing barrel?
The only thing I can think of is, find a barrel for S&W gun that could fit, and see if a gunsmith could retrofit it. That's pretty dicey though, IMO. I wouldn't want anybody chopping my gun up like that.