For equally spaced discretization, let h=step-size. For not equally spaced let h=max{h1,h2,h3}. hi is step-size in ith direction. Then find an operator which produces the error between numerical and exact as O(h^n), then the operator is nth order accurate. For example, central difference scheme is second-order accurate

Thank you prof. for your very good suggestion. Please suggest some journals or books for how to find the operator in case of unequal space data.  Thank you very much.

This lecture notes deals with 2D discretization. You could see, hx, hy. Please refer page 83. The same can be extended to 3D.

http://www4.ncsu.edu/~zhilin/TEACHING/MA402/chapt5.pdf

Thank you Prof. for your good suggestion.

Dear Dr

Some of my Papers

1- A. R. Seadawy, Fractional solitary wave solutions of the

nonlinear higher-order extended KdV equation in a stratified

 shear flow: Part I, Comp. and Math. Appl. \textbf{70} (2015)

 345–352.

2- Khater, A. H., Callebaut D. K., Malfliet, W. and Seadawy A. R., Nonlinear Dispersive

Rayleigh-Taylor Instabilities in Magnetohydro-dynamic Flows,

Physica Scripta, 64 (2001) 533-547.

3- Khater, A. H., Callebaut D.

K. and Seadawy A. R., "Nonlinear Dispersive Kelvin-Helmholtz

Instabilities in Magnetohydrodynamic Flows" Physica Scripta, 67

(2003) 340-349.

4- A.R. Seadawy, Exact solutions of a two-dimensional

nonlinear Schrodinger equation, Appl. Math. Lett. 25 (2012) 687.

5- A.R. Seadawy, Stability analysis for Zakharov-Kuznetsov equation of weakly

nonlinear ion-acoustic waves in a plasma, Computers and

Mathematics with Applications 67 (2014) 172-180.

6- A.R. Seadawy, Stability analysis for two-dimensional ion-acoustic waves in

quantum plasmas, PHYSICS OF PLASMAS 21 (2014) 052107.

7- Helal, M. A. and  Seadawy A. R., Variational method for the

derivative nonlinear Schrodinger equation with computational

applications, Physica Scripta, 80, (2009) 350-360.

8- Helal, M. A. and  Seadawy A. R., Exact

soliton solutions of an D-dimensional nonlinear Schrodinger

equation with damping and diffusive terms, Z. Angew. Math. Phys.

(ZAMP) 62 (2011), 839-847.

9-  Khater, A. H., Callebaut D. K. and Seadawy A. R., General

soliton solutions of an n-dimensional Complex Ginzburg-Landau

equation, Physica Scripta, Vol. 62 (2000) 353-357.

10- Khater, A. H., Helal M. A. and Seadawy A. R.,

General soliton solutions of n-dimensional nonlinear Schrodinger

equation" IL Nuovo Cimento 115B, (2000) 1303-1312.

11-  Khater, A. H., Callebaut D. K., Helal, M. A. and  Seadawy A.

R., Variational Method for the Nonlinear Dynamics of an Elliptic

Magnetic Stagnation Line, The European Physical Journal  D, 39,

(2006) 237-245.

12-  Khater, A. H., Callebaut D. K.,

Helal, M. A. and  Seadawy A. R., General Soliton Solutions for

Nonlinear Dispersive Waves in Convective Type Instabilities,

Physica Scripta, 74, (2006) 384.

13- Seadawy A. R., Three-dimensional nonlinear modified Zakharov–Kuznetsov

equation of ion-acoustic waves in a magnetized plasma, Computers

and Mathematics with Applications 71 (2016) 201-212.

14- M.A. Helal and A.R. Seadawy, Benjamin-Feir-instability in

nonlinear dispersive waves, Computers and Mathematics with

Applications 64 (2012) 3557-3568.

15- Seadawy, A.R., and El-Rashidy, K., Traveling wave solutions for some coupled nonlinear

evolution equations by using the direct algebraic method, Math.

and Comp. model. \textbf{57} (2013) 1371.

16- A. R. Seadawy, New exact solutions for the KdV equation with higher  order nonlinearity

by using the variational method, Comp. and Math. Appl. 62 (2011)

3741-3755.

17- Seadawy A. R, Stability analysis solutions for nonlinear

three-dimensional modified Korteweg-de Vries-Zakharov-Kuznetsov

equation in a magnetized electron-positron plasma" Physica A:

Statistical Mechanics and its Applications  Physica A 455 (2016)

44-51.

18- A.R. Seadawy, Nonlinear wave solutions of the three-dimensional

Zakharov–Kuznetsov–Burgers equation in dusty plasma, Physica A

439 (2015) 124–131.

19- A.R. Seadawy, Stability analysis of traveling wave

solutions for generalized coupled nonlinear KdV equations, Appl.

Math. Inf. Sci. 10 (1) (2016) 209–214.

Thank you Sir for your suggestions.