$\cos 3\theta$ equals:

Triple-angle for cosine: $\cos 3\theta = 4\cos^3\theta - 3\cos\theta$. Derivation: $\cos 3\theta = \cos(2\theta + \theta) = \cos 2\theta \cos\theta - \sin 2\theta \sin\theta = (2\cos^2\theta - 1)\cos\theta - 2\sin\theta\cos\theta \cdot \sin\theta = 2\cos^3\theta - \cos\theta - 2\sin^2\theta\cos\theta = 2\cos^3\theta - \cos\theta - 2(1-\cos^2\theta)\cos\theta = 4\cos^3\theta - 3\cos\theta$. Companion: $\sin 3\theta = 3\sin\theta - 4\sin^3\theta$ (option C — common trap). Mnemonic: in both, the coefficients are 3 and 4, and the cubed term has the same trig function as the angle outside.