median of 3 (or bigger odd number) values is generally interpretted as you take the middle one when sorted in order of magnitude, without any averaging. This is sensible as it automatically ignores extremes and spikes values on any one channel. For even number you do average the middle pair. No difference between mean and median if you only have 2 values! A median filter also be applied to multiple samples over a time period, linear/ averaging filters tend to turn spikes into bumps.