You can use the Lighten and Darken blend modes on your layers for Maximum and Minimum.
Averaging frames (Mean) would be a little more complicated. You'd first have to figure out how many layers you're going to average together, then use an effect to reduce the intensity of the layers appropriately. For example, with 10 layers you'd need each layer to have 1/10th its original value. You could do this with a Levels effect by lowering the Output White to 1/10th its default value or with a Channel > Solid Composite effect with 'Color' set to Black, 'Opacity' set to 90% and 'Blending Mode' set to Multiply. Then you'd set all 10 layers to the 'Add' blend mode. So you've essentially added all 10 layers together and divided the result by 10 (averaging the colors).
If you're working at 32bpc it'd be a little easier as you could 'Add' all the layers together and put a layer at the top of the stack set to the 'Divide' blend mode with a color value of 10 in R, G and B (for 10 layers).
There is no way to find the Median value of several layers in After Effects without a 3rd party plug-in. I believe Re:Vision Effects' De:Noise plug-in allows you to perform a temporal Median operation but I think only across 3 frames from the same layer.