dictionary_example = {'1234': {'choice': 0, 'choice_set': {(0): {'A': 100,
    'B': 200, 'C': 300}, (1): {'A': 200, 'B': 300, 'C': 300}, (2): {'A': 
    500, 'B': 300, 'C': 300}}}, '234': {'choice': 1, 'choice_set': {(0): {
    'A': 100, 'B': 400}, (1): {'A': 100, 'B': 300, 'C': 1000}}}, '1876': {
    'choice': 2, 'choice_set': {(0): {'A': 100, 'B': 400, 'C': 300}, (1): {
    'A': 100, 'B': 300, 'C': 1000}, (2): {'A': 600, 'B': 200, 'C': 100}}}}
df = pd.read_json(json.dumps(dictionary_example)).T

def to_s(r):
    return pd.read_json(json.dumps(r)).unstack()
flattened_choice_set = df['choice_set'].apply(to_s)
store dictionary in pandas dataframe