Calculate mean of several elevation grids?

I dont understand the syntax of the raster calculator. I would eventually like to calculate the variance of several elevation maps but, to start with, an example of how to calculate the mean of three DEMs would work. (I know you can do this in the Analysis/Combine tool, but I would like to see an example of the syntax in the raster calculator). Say I have:
dem1, dem2, and dem3  ....each a gridded elevation layer (with same grid size and projection, but different extents).
I would like to do:
mean = (dem1+dem2+dem3)/3.

How is that entered in the Formula box? How do I link the variable names (mean, dem1, ...) with the layers?

Thanks!
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