Do the columns represent the dimensions, so column 1 is x values, column 2 is y values and column 3 is z values? Then, a L*M*N array wouldn't make a lot of sense here as you don't have a uniform mesh but rather individual sample points. You can access them by x=myarray(:,1); and so on.
In case I got the meaning of the columns wrong, the function reshape might help you. slice is no array transformation but a plotting method.
