Here is another approach to maintaining the same plot style for each graph independent of which set of checkboxes are checked:
Manipulate[
Plot[Evaluate[
choices /. {1 -> Sin[x], 2 -> Cos[x], 3 -> BesselJ[3, x]}], {x, 0,
10},
PlotRange -> {All, {-1.5, 1.5}},
PlotStyle -> {Directive[Red, Thick], Directive[Blue, Dashed],
Directive[Green, Medium]}[[choices]]], {{choices, {1, 2, 3}}, {1,
2, 3}, CheckboxBar}]