attractors of Mercury's spin-orbit relation
Over enough time there is a tendency of all orbiting objects to fall into resonance with the object they're orbiting.   1:1 being is most common.

1:1 usually called Tidal Locking, is quite common. The Moon to the Earth.   Pluto and Charon to each other.  All four of the Galilean moons are tidally locked to Jupiter, likely others.

Mercury's 3:2 ratio is more unusual but not necessarily uncommon, especially for a planet with an eccentric orbit.

See Here 

An eccentric orbit likely helps create a spin-orbit resonance other than 1:1 and resonance other than 3:2 should certainly be possible (the Wiki article implies that) though there are no known examples that I know of.

For a really crazy orbital spin - check this out:  http://www.businessinsider.com/pluto-moons-weird-behavior-2015-6