str.replace("USA", "United States of America")
str.replace("CAN", "Canada")
str.replace("BHM", "Bahamas")
str.replace("CUB", "Cuba")
str.replace("HAI", "Haiti")
str.replace("DOM", "Dominican Republic")
str.replace("JAM", "Jamaica")

mapofcodes = {'USA': 'United States of America', ....}
for word in mystring.split():
    finalstr += mapofcodes.get(word, word)

import re

COUNTRIES = {'USA': 'United States of America', 'CAN': 'Canada'}

def repl(m):
    country_code = m.group(1)
    return COUNTRIES.get(country_code, country_code)

p = re.compile(r'([A-Z]{3})')
my_string = p.sub(repl, my_string)