Why am I getting 'name is not defined' - beginner : I'm very new at coding and struggling to understand why this gives me name 'month' is not defined

I'm trying to convert input(10, 5, 2017) to October 5, 2017 for example

    def problem3_3(month, day, year):
    months_tuple = ('January', 'February', 'March','April', 'May', 'June', 
    'July', 'August', 'September', 'October', 'November', 'December')

    A = months_tuple[month-1]
    B = str(day)
    C = str(year)

    print("A", +"B,"+"C")