(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" (and immediately agrees to an offer for more) (Star Trek Generations)
Although it hosts a bat'leth tournament and might 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.