A 3 x 512 array is a 2D array, not a 3D array. Assuming you've read your ASCII table into an array in your program (perhaps using csvread or dlmread), you could do something like (untested)
[rows columns] = size(array3x512);
for row = 1 : rows
x = array3x512(row, 1);
y = array3x512(row, 2);
z = array3x512(row, 3);
lut2D(y, z) = x;
end
Then later, when you have some arbitrary y and z that you want the x for, you just say
x = lut2d(y,z);
Of course you'll have to round all your indexes to integers - that's required if you want to do a look up table instead of interpolation. You may also have to figure out what to do if every single y and z pair is not represented.
