Case 3 Achievement:
When you first begin case 3 you will need to answer the phone and then move into the next area, in the next area pick up the phone in the yellow booth when it rings and then take the scooter next to it. Once you have the scooter follow the path out of the area and and after a few cutscenes you will reach the town. The achievement won't pop right away so when you reach town speak with lobster cop and he will be added to the journal, from there close your journal and you should gain the achievement.
If you need more help with this you can view my guide below which starts at this achievement: