What is a three-caliber ????
The only thing I can think of is a .357 mag and it technically will shoot more than 3 different cartridges tho they are all the same caliber.
The .22 jet plus several .22 rimfires (short long and long rifle)
There was also one with two cylinders for .22 Mag, .22 WRF, and all the regular .22 rimfires.
I suspect the writer was just an idiot tho.
Technically, it would be a 3" bore.