loop.py

letters = ['A', 'B', 'C']
nb_letters = len(letters)
total = 100
step = total / nb_letters

index = 0
results = []
for i in range(1, total + 1):
    if index == nb_letters:
        index -= 1
    results.append(letters[index])
    if i % step == 0:
        index += 1

print results