What is a triple star system?
A triple star system has three stars orbiting around their common centre of mass.
Stars come in a range of configurations. Stars like our Sun are the only star in the system. Many stars are binary stars with two stars orbiting their centre of mass. Solar systems can have more than two stars, but of these a triple system with three stars is the most common.
A good example of a triple star system is our nearest neighbour Alpha Centauri. This consists of a binary star system consisting of Alpha Centauri A and Alpha Centauri B. These are orbiting quite close to each other. The third star Proxima Centauri is smaller and orbits the other two at about 1,000 times the distance between the binary pair.
Having a third star orbiting a binary star system is quite stable. If the distances of the three stars are quite close then the orbits are unstable.
Instabilities can lead to one of the stars being ejected due to the gravity of the other two.