# Describing 3-slit interference patterns

I'm typing up some physics notes about wave interference patterns and want to include a graph of the wave produced by a 3-slit experiment:

Does anyone know what formula produces this kind of curve? I've been searching for a while but haven't found anything. I know it must be some kind of sine wave.

Try $\cos(x) + \frac12 \cos(2x)$ for example.
In my vision of this problem, the slits are at $(x,y) = (0,0)$, $(s,0)$ and $(-s,0)$. The screen is at $y^2=1000$. The wave length is $2 \pi$. (The Animate[] command is useful for exploring the parameter space a little bit.)