[tex]\cos (3x) = 4\cos^3(x) -3 \cos(x)[/tex]
The way to remember it in Hokkien (a local dialect commonly spoken in Singapore, Taiwan and southern parts of China):
Cor Sah ($1.30) = Si Cor Sah ($4.30) – Sah Cor ($3).
You can actually do it in Chinese, if you associate the chinese word for dollar “Kuai” with cosine. It’s just that the Hokkien pronunciation is much closer.
Source: From a friend who does not remember where he heard it from.
PS: Is there a mnemonic for remembering how to spell the word mnemonic?