An analogue to covariance for three variables. For three variables x, y, and z with values x_i, y_i, z_i, i=1,…,n, the comvariance is com(x,y,z) = _i=1…n(x_i-m(x))(y_i-m(y))(z_i-m(z)), where m(•) is the mean of the values in its argument. Due to Deardorff (1982).