var person1 = {
	firstname: 'Иван',
	lastname: 'Иванов',
	city: 'Киев',
	phone: '+38099-999-99-99',
	birthday: '01-01-1991'
},
person2 = {
	firstname: 'Петр',
	lastname: 'Петров',
	city: 'Москва',
	phone: '+7819-666-66-66',
	birthday: '02-02-1992'
},
person3 = {
	firstname: 'Тихон',
	lastname: 'Тихонов',
	city: 'Петрозаводск',
	phone: '+38077-777-77-77',
	birthday: '03-03-1993'
};

//1
person1.middlename = 'Иванович';
person3.middlename = 'Тихонович';

//2
var users = [];
users = users.concat(person1,person2,person3);

//3

for (i = 0; i <= users.length; i++) {
	for (var key in users[i]) {
		if (users[i].middlename) {
			users[i].fullName = users[i].firstname + ' ' + users[i].middlename + ' ' + users[i].lastname;
		}
		if (!users[i].middlename) {
			users[i].fullName = users[i].firstname + ' ' + users[i].lastname;
		}

	}
}