documentclass[convert]{standalone}
usepackage{pgfplots, pgfplotstable}
begin{document}

pgfdeclareplotmark{a}{pgfuseplotmark{*}}
pgfdeclareplotmark{b}{pgfuseplotmark{triangle*}}
pgfdeclareplotmark{c}{pgfuseplotmark{square*}}

pgfplotstableread{%
x y label
0.1  0.15 a
0.45 0.27 c
0.02 0.17 a
0.06 0.1  a
0.9  0.5  b
0.5  0.3  c
0.85 0.52 b
0.12 0.05 a
0.73 0.45 b
0.53 0.25 c
0.76 0.5  b
0.55 0.32 c
}{tab}

begin{tikzpicture}
begin{axis}[legend pos=south east]
  addplot[%
    scatter,
    only marks,
%    visualization depends on=value thisrow{label}aslabel,
%    scatter/@pre marker code/.append style={/tikz/mark=label},
  ]
  table[x=x,y=y]{tab};
  legend{Class 1,Class 2,Class 3}
end{axis}
end{tikzpicture}
begin{tikzpicture}
begin{axis}[legend pos=south east]
  addplot[%
    scatter,
    only marks,
    visualization depends on=value thisrow{label}aslabel,
    scatter/@pre marker code/.append style={/tikz/mark=label},
  ]
  table[x=x,y=y]{tab};
  legend{Class 1,Class 2,Class 3}
  end{axis}
end{tikzpicture}
begin{tikzpicture}
begin{axis}[legend pos=south east]
  addplot[%
  scatter,
  only marks,
  visualization depends on=value thisrow{label}aslabel,
  scatter/@pre marker code/.append style={/tikz/mark=label},
  ]
  table[x=x,y=y]{
    x y label
    0.1  0.15 a
    0.45 0.27 c
    0.02 0.17 a
    0.06 0.1  a
    0.9  0.5  b
    0.5  0.3  c
    0.85 0.52 b
    0.12 0.05 a
    0.73 0.45 b
    0.53 0.25 c
    0.76 0.5  b
    0.55 0.32 c
  };
  legend{Class 1,Class 2,Class 3}
end{axis}
end{tikzpicture}
end{document}