$Version
(* 11.3.0 for Microsoft Windows (64-bit) (March 7, 2018) *)
f[x_, t_] := Sin[2 x]*Cos[t]^2;
Plot[f[#, t] & /@ {Pi/4, Pi/6, Pi/12} // Evaluate, {t, 0, 3}, PlotStyle -> {Black, {Black, Dashed}, {Black,
AbsoluteDashing[{2, 4}]}}, PlotLegends -> LineLegend[{"\!\(\*FractionBox[\(Pi\), \(4\)]\)", "\!\(\*FractionBox[\(Pi\), \(6\)]\)",
"\!\(\*FractionBox[\(Pi\), \(12\)]\)"}, LegendLabel -> "Label", LegendFunction -> (Framed[#, RoundingRadius -> 5] &),
LegendMargins -> 5], AxesLabel -> {"\!\(\*FractionBox[\(Gt\), \(Pi\)]\)", "CAB"}]
.
Regards,MI