H[f_]=(1/3)+(1/3)*Exp[-I*2*Pi*f]+(1/3)*Exp[-I*4*Pi*f];

Simplify[(((ComplexExpand[Abs[H[f]]]))^2)]

Plot[((Abs[.33+.33*Exp[-I*2*π*f]+.33*Exp[-I*4*π*f]])^2),{f,-.5,.5},AxesLabel->{"f","(Subscript[S, Y])(f)"}, PlotLabel->"PSD of a Moving Average RP", GridLines->Automatic, PlotStyle->Thick]

(∫_(-.5)^(.5))((Abs[.3333+.3333*Exp[-I*2*π*f]+.3333*Exp[-I*4*π*f]])^2)\[DifferentialD]f