The peaks arise from envelopes that arise when the function value nearly repeats. The peaks are at $\pm {\rm cos}(x)\pm {\rm cos}(\alpha x)\pm {\rm cos}(\beta x)$ for this case. More details can be found here.

The peaks arise from envelopes that arise when the function value nearly repeats. The peaks are at the roots of the implicit equation $\pm {\rm cos}(x)\pm {\rm cos}(\alpha x)\pm {\rm cos}(\beta x)=0$ for the case of the function $f(x)= {\rm cos}(x)+ {\rm cos}(\alpha x)+ {\rm cos}(\beta x)=0$. More details can be found here.