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.

The wave looks a lot like an in-phase superposition of two waves, one with twice the frequency of the other.

Try $\cos(x) + \frac12 \cos(2x)$ for example.

How about something like the following Mathematica commands

f[s_] := Plot[Cos[Sqrt[x^2 + 1000]] + Cos[Sqrt[(x + s)^2 + 1000]] + Cos[Sqrt[(x - s)^2 + 1000]], {x, -8 Pi, 8 Pi}]; f[4.7]

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.)

It was not clear to me from the question whether the picture was supposed to be of the amplitude or the intensity of the interference pattern. I've given the amplitude above. For the intensity, one should square the sum of cosines.