(covers information from several alternate timelines)
Forcas III was the origin of a drink that Guinan served in Ten Forward on the USS Enterprise-D. When Data tried this drink, during one of his first experiences with his emotion chip, he described the Forcas III drink as "revolting". (Star Trek Generations)
Although it hosts a bat'leth tournament and migth therefore be part of the Klingon Empire, the location and affiliation of Forcas III has never been stated on screen.
According to the Star Trek: Star Charts (pg. 64), the Forcas system was located in or near to Klingon space, in the Beta Quadrant. Primary was an Class K star with a magnitude of +5, which was the same brightness as Sol.
Forcas III is mentioned as one of the Albino's past whereabouts in the novel Excelsior: Forged in Fire. Taking place in 2409, the computer game Star Trek Online requires the player to participate at a bat'leth tournament on Forcas III for the House of Martok.