Hello there! We can't give much help on the hidden achievements -- that is why they are hidden! Here's what we can share:
1. One of them is VERY early in the game (if you get it, you will get it before Lazren returns the first time).
2. One of them is available the entire game, but it's very difficult to find. You definitely need to think outside the box.
3. One of them is only available in Chapter 3 (that is, after Chastity shows up and starts testing you).
Of these, we know that people have found #1 and #3, but we don't know if anyone has found #2. The third one (#3) seems to be the easiest to find.