dst = 70, 20, 10
vls = 'a', 'b', 'c'
picks = [v for v, d in zip(vls, dst) for _ in range(d)]
for _ in range(12):
    print(random.choice(picks), end=' ')
for _ in range(12):
    print(random.choice(picks), end=' ')
for _ in range(12):
    pass
выбор на основе процентного взвешивания