Why is Fe3+ ion more stable than Fe2+ ion?
Your question is very much appreciable.
The underlying principle involved in this is “Half-filled and fully-filled orbitals are extra stable”.
The d subshell has five orbitals, which can accommodate a maximum of 10 electrons.Fe+3 (3d5) has half filled orbitals which provide extra stability to it because of symmetrical configuration. While Fe+2(3d6) has one electron more than the symmetrical arrangement. That’s why Fe+3 is more stable than Fe+2.
Hope, this answers your question.