I assume your variables are vectors. If they are gridded, it is only necessary to use the reshape function to form them into matrices.
If they are not gridded, something like this will work:
x = rand(20, 1);
y = rand(20, 1);
z = rand(20, 1);
xv = linspace(min(x), max(x), 50);
yv = linspace(min(y), max(y), 50);
[X,Y] = ndgrid(xv, yv);
Z = griddata(x, y, z, X, Y);
contourf(X, Y, Z)
Experiment to get the result you want.