If you get stuck in Act 3 and don't know what to do next, check out these checkpoints.
Have you visited Meowy's Mporium and do his promotional campaign?
Have you got into the Establishment?
Have you prepared the paintstripper and used it?
After using the paintstripper: ? .
Talk to Cosmo
Did you get into Edgar's locker?
Have you talked to Lady Scarlett about she being a Catulet? Also: ?
Show Scarlett the photo you found from Edgar's locker.
Break into Bartholomew's safe. After that: ?
Look at the birth certificate.
Go talk to Scarlett and visit the mausoleum.