We discussed this very gun back in 2014. At that time, Bill Murphy, who has followed the market as much as anyone, estimated its value in the $7500 - $8500 range. I would agree it would probably be valued in that same range as well today.
Here's a link to that previous discussion. There's another link to the pictures of this 3-barrel set that are in the All Albums section within this link.
http://parkerguns.org/forums/showthr...ght=Stub+Brown
I have seen a very few 3-barrel sets through the years. I seem to recall they were bi-gauge sets -- 20/20/16 and 28/28/.410. Yours with 3 sets of 20ga. bbls. may be unique.
Threaded Mode