function unary3dBy( arrays, shape, fcn, clbk ) {
	var thisArg;
	var S0;
	var S1;
	var S2;
	var i0;
	var i1;
	var i2;
	var x0;
	var x1;
	var y0;
	var y1;
	var x;
	var y;
	var v;

	S0 = shape[ 2 ];
	S1 = shape[ 1 ];
	S2 = shape[ 0 ];
	if ( S0 <= 0 || S1 <= 0 || S2 <= 0 ) {
		return;
	}
	if ( arguments.length > 4 ) {
		thisArg = arguments[ 4 ];
	}
	x = arrays[ 0 ];
	y = arrays[ 1 ];
	for ( i2 = 0; i2 < S2; i2++ ) {
		x1 = x[ i2 ];
		y1 = y[ i2 ];
		for ( i1 = 0; i1 < S1; i1++ ) {
			x0 = x1[ i1 ];
			y0 = y1[ i1 ];
			for ( i0 = 0; i0 < S0; i0++ ) {
				v = clbk.call( thisArg, x0[ i0 ], [ i2, i1, i0 ], [ x, y ] );
				if ( v !== void 0 ) {
					y0[ i0 ] = fcn( v );
				}
			}
		}
	}
}
Applies a unary function to each element retrieved from a three-dimensional nested input array according to a callback function and assigns results to elements in a three-dimensional nested output array.

## Notes

-   The function assumes that the input and output arrays have the same shape.

@param {ArrayLikeObject<Array>} arrays - array-like object containing one input nested array and one output nested array
@param {NonNegativeIntegerArray} shape - array shape
@param {Function} fcn - unary function to apply to callback return values
@param {Callback} clbk - callback function
@param {*} [thisArg] - callback execution context
@returns {void}

@example
var ones3d = require( '@stdlib/array/base/ones3d' );
var zeros3d = require( '@stdlib/array/base/zeros3d' );

function accessor( v ) {
    return v - 2.0;
}

function scale( x ) {
    return x * 10.0;
}

var shape = [ 1, 2, 2 ];

var x = ones3d( shape );
var y = zeros3d( shape );

unary3dBy( [ x, y ], shape, scale, accessor );

console.log( y );
// => [ [ [ -10.0, -10.0 ], [ -10.0, -10.0 ] ] ]