Manipulate[Plot[{0.5 x+1,x,2 x-2},{x,-1,5},PlotRange->{-1,5},AspectRatio->1,PlotStyle->{Opacity[a],Opacity[b],Opacity[c]},Epilog->{Text[If[a==1,"f(x)",""],{4.5,2.7}],Text[If[b==1,"y=x",""],{4.5,4}],Text[If[c==1,"g(x)",""],{3,4.5}]}],{{a,1,"f(x)"},{1,0}},{{b,1,"y=x"},{1,0}},{{c,1,"g(x)"},{1,0}},ControlPlacement->Left,FrameMargins->0]