Plot[ { Cos[x], (x^2)-((π^2)/4),((3(π^2))/4) - (x^2)}, {x, -3, 3}]

Plot[{Cos[x],(x^2)-((π^2)/4),((3 (π^2))/4)-(x^2)},{x,-3,3},PlotStyle->Directive[RGBColor[0.22`,0.05`,1.`],AbsoluteThickness[4.]]]