Step 1. Talk to Rookie, he will close the vault. Then he forgets the combination.
Step 2. Walk into the Gift Shop's Manager Room, turn right until you see a sofa, click underneath it and collect the paperclip and the disk.
Step 3. Go to the computer, switch it on and insert the disk. Go to My Files, Combination Number. It will show you some numbers, memorise them and go back down to the vault.
Step 4. Put the combination in and open the vault. Go inside and you will see all the coins! ...on the ceiling. Talk to Rookie, then G should call you on your spy phone, meet him in the HQ. He will show you a video, after talk to G and he will give you a set of keys, click the key in the top left corner.
Step 5. Collect the flashlight on the drawers, then go back to the Gift Shop Manager's Room, open the door that says "To Roof".
Step 6. Get the wrench from your spy phone and open the power box, then put the paper clip inside it. Pick up the piece of fur that is next to the drain and put it in your inventory.
Step 8. Go inside the Night Club, use the flashlight to navigate your way through the Night Club to the Boiler Room. In the Boiler Room there is a fuse box, you need to solve the puzzle to switch the power back on.
Step 9. Report back to the HQ and talk to G.
Congratulations! You completed Mission 3, claim your card and reward!