I don't quite understand your data, is B a function of A (or visa versa)? Also, those arrays appear to be 1D, not 3D. Can you clarify this?
In your function call
yy = spline(x, Y, xx)
Y is the dependent variable which you are interpolating: Y is a function of x and the result of the above function call is to return the value of Y at xx. As an example in one dimension, try
x = linspace(0., 2.*pi, 100);
Y = sin(x);
% What is the value of Y (or sin(x)) at pi/2?
xx = pi/2.;
yy = spline(x, Y, xx); % This should result in yy = 1.000
Check out the spline documentation for more information and examples of using this function.
Now this function is only for 1D fitting, and is (I presume) equivalent to yy = interp1(x, Y, xx, 'spline'). If you want to do a three dimensional lookup, you'll have to use interp3, which generalises the above example to 3D. So rather than just one independent coordinate x, we have two more, y, and z and three coordinates for the point at which we want to perform the look up: xx, yy and zz. The function you are interpolating must be a 3D function of the coordinates (x, y, z). Try, as an example:
x = linspace(-1., 1., 100); y = x; z = x;
[X, Y, Z] = meshgrid(x, y, z);
s = exp(-sqrt(X.^2 + Y.^2 + Z.^2));
sinterp = interp3(x, y, z, s, 0., 0., 0.) % Should give sinterp = 0.9827
