Can I serve a 3 day notice because the tenant put a lock on the door and I have no accss?

Yes, you can serve a 3-day notice if a tenant has changed the locks and is denying you access to the property, as this may violate the lease agreement or local landlord-tenant laws. However, it's important to check your local regulations to ensure you follow the proper legal procedures for notice and lock changes. Additionally, consider documenting the situation and attempting to communicate with the tenant before taking further action. Consulting with a legal professional is advisable to ensure compliance with applicable laws.