The clustering method of panel data is studied. The 2nd order difference is used to substitute for the approximate
2nd derivative based on the grey convex relation for two dimension. The half positive characteristic of Hessian matrix is
used to define the convexity degree. The similarity degree of objects is characterized by the convexity, the grey convex
relation degree for three dimension is presented, and the properties of the grey convex relation degree for three dimension
are discussed. An example is given to illustrate that the grey convex relation degree for three dimension can better reflect the
correlation degree for three dimension panel data.