fitting to multidimensional data

I am using fttype and fit functions to fit a costume function to my data. my independent variable has 3 dimension.

this is my code: ft = fittype( 'expo_space(x,a1,b1,c1,d1,e)' ); f = fit( [x(:,1),x(:,2),x(:,3)], output1, ft, 'StartPoint', [1, 1, 1, 1, 1] );

expo_space is my costume function.

I get this error :

Error using ==> fittype.fittype at 418 Expression expo_space(x,a1,b1,c1,d1,e) is not a valid MATLAB expression, has non-scalar coefficients, or cannot be evaluated: Error in fittype expression ==> expo_space(x,a1,b1,c1,d1,e) ??? Attempted to access x(:,2); index out of bounds because size(x)=[2,1].

Error in ==> para_space_expo at 160 ft = fittype( 'expo_space(x,a1,b1,c1,d1,e)' );

how can i use fittype to work with multi dimentsiona data?