Why Fe 3+ is more common than Fe 2+
The electron configuration of  $\ce{Fe(II)}$ is $1s^22s^22p^6 3s^23p^6 3d^6$.

The electron configuration of  $\ce{Fe(III)}$ is $1s^22s^22p^6 3s^23p^6 3d^5$.

The half filled  orbital $3d$ is more stable than the same orbital filled with 6 electrons. So,  $\ce{Fe(III)} $ ion is more common than $\ce{Fe(II)}$.