If we're gonna go totally Von Daniken on this, we should take into account the fact that the Orion aliens probably* landed their spaceships in groups of 3, hence the need for 3 henges, whilst the Martians obviously only landed in single craft. Hence on Mars, single circular structures abound.

*sod 'probably'. When making claims of this nature, it is better to have absolute facts passed on by channeling of the spirits of dead martians. So:- The Orion aliens *did* land their spaceships in groups of 3. They told me so ;)