let surname = prompt('Фамилия');
let firstName = prompt('Имя');
let middleName = prompt('Отчество');

if(!surname) {
	surname = 'Гончаренко';
}
if(!firstName) {
	firstName = 'Сергей';
}
if(!middleName) {
	middleName = 'Николаевич'
}

alert(`${surname} ${firstName} ${middleName}`);