You must be on update 0.7 or above(it's not available for Android yet).
On level 8 find clothes in some area. You will notice the left wall is looking little unusual. Jump there from rope to find the secret path and first missing girl.
Second missing girl is in level 15 where you will find artifact earlier. You have crawl on the other side and interact with small cave to get on the other side.
Third missing is in level 13. I think she's the behind four consecutive ruby ore.