I got 7* Ant-Man... 3 times in a row. Three times.
Considering that the basic pool is approximately large of 50 champs that's less that 1 chance out of 100 000. And the fun fact that is before these 3 last crystals, my Ant-Man was already duped.

So I think that whatever you opened from your 3 last crystals, you are waaay luckier than me. I have 29 7* and 3 dupes btw.