function callbackBody( ch1, ch2, ch3 ) {
	if ( isComplexChar( ch1 ) || isComplexChar( ch2 ) || isComplexChar( ch3 ) ) { // eslint-disable-line max-len
		return '// ...';
	}
	if ( ch1 === 'x' || ch2 === 'x' || ch3 === 'x' ) {
		return 'return x && y;';
	}
	return 'return x + y;';
}
Returns a callback body based on callback input and output data types.

@private
@param {string} ch1 - one-letter character abbreviation for first input argument
@param {string} ch2 - one-letter character abbreviation for second input argument
@param {string} ch3 - one-letter character abbreviation for output value
@returns {string} callback body