documentclass{scrartcl}
DeclareNewSectionCommand[
    afterskip=1.5ex plus .2ex,
    beforeskip=-3.25ex plus -1ex minus -.2ex,
    indent=0pt,
    level=2,
    font=usekomafont{subsection},
    tocindent=1.5em,
    tocnumwidth=2.3em,
    counterwithin=section,
    style=section]{subphase}

DeclareNewSectionCommand[
    afterskip=1.5ex plus .2ex,
    beforeskip=-3.25ex plus -1ex minus -.2ex,
    indent=0pt,
    level=3,
    font=usekomafont{subsection},
    tocindent=2.0em,
    tocnumwidth=2.3em,
    counterwithin=subphase,
    style=section]{task}

begin{document}
 section{A}
 subphase{Phase}
 task{Task}
 task{Task}
 task{Task}
 subphase{Phase}
 task{Task}
 task{Task}
 task{Task}
end{document}

! TeX capacity exceeded, sorry [input stack size=5000].
thesubphase ...sname thescr@local@counterwithin
                                                  endcsname .arabic {subph...
l.26  subphase{Phase}