You need first to shuffle the list and then select 3 elements from it.
>>> import random
>>> t=[i for i in range(15)]
>>> t
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]
>>> print(random.shuffle(t))
None
>>> random.shuffle(t)
>>> t
[6, 9, 13, 8, 5, 12, 7, 10, 1, 2, 14, 0, 4, 11, 3]
>>> t[:3]
[6, 9, 13]
>>>