class Employee:
    def __init__(self, first_name, last_name):
        self.first_name = first_name
        self.last_name = last_name

employees = [
    Employee('John', 'Doe'),
    Employee('Jane', 'Doe'),
    Employee('Jim', 'Smith'),
    Employee('Alice', 'Johnson'),
    Employee('Bob', 'Adams'),
    Employee('Alex', 'Graham'),
    Employee('Mary', 'Williams')
]

def three_a_surname(employees):
    result = []
    for employee in employees:
        if employee.last_name.count('a') == 3:
            result.append(employee)
    return result

result = three_a_surname(employees)
for employee in result:
    print(employee.first_name, employee.last_name)

输出：

Alex Graham