A compact Riemann surface of genus g ≥ 2 is said to be extremal if it
admits an extremal disk, a disk of the maximal radius determined by g.
If g = 2 or g ≥ 4, it is known that how many extremal disks an extremal surface of genus g can admit.
In the present paper we deal with the case of g = 3. Considering the
side-pairing patterns of the fundamental polygons, we show that extremal
surfaces of genus 3 admit at most two extremal disks and that 16 surfaces admit
exactly two. Also we describe the group of automorphisms and hyperelliptic
surfaces.