At the intersection of 3 galactic empires (one of which is human) There is a planet guarded by all 3 because that race is too warlike
Your description reminds me of Jack Vance's Planet of Adventure. Could it be the novel you're referring to?
From the Planet-of-Adventure Wikipedia entry:
In the novels, Tschai is a planet [...], 212 light-years from Earth. It is populated by three alien, mutually hostile species; the displaced, native Pnume; and various human races, some of whom live as slaves or clients of the aliens.