TECHNICALLY (on average)

Every 3-4 you buy would yield a 4 star.
Every 16-17 yields a 5 star
Every 200 yields a 6 star

So not getting all 3 stars with 10 gacha is a bit unlucky. About the same chance as flipping 3 coins and getting the same side (you did not pick) all 3 times. Looking at it that way it's unlucky but not a super rare occurrence. If you get a 5 star on the other hand, it could be considered lucky, but not really either. 6 star... I don't even need to say.