Sketch one cycle of the cosine function y= – cos 3θ

Hmmm same as the other you posted... is the graph of y= cos(θ), but with a negative Amplitude of -1, and a B component of 3

let's check the period again  

so, is the same graph, with a period of   and the negative amplitude will simply, retain the same midline, but the graph will be turned upside-down