....

JFG_JG_dict = {}
for index2, row2 in median_JFG_JG.iterrows():
    if pd.isnull(row2['base_salary_75th [LOCAL]'])==False:
        JFG_JG_dict[(row2['COUNTRY_CODE'], row2['Job Family Group'], row2['JG'])] = row2['base_salary_75th [LOCAL]']

...

SOME_FINAL_DEFAULT_VALUE = None # jakas sensowna wartosc?

for index, row in dataset.iterrows():
    if pd.isnull(row['base_salary_75th [LOCAL]']):
        dataset.loc[index,'base_salary_75th [LOCAL]'] = JFG_JG_dict.get((ctry_cd, jfg, jg), False) or JFG_JG_dict.get((jfg, jg), SOME_FINAL_DEFAULT_VALUE)