Occurence of QuickBooks Error Code 3003 can be attributed to the failure of synchronization of the database. 

Causes for occurrence of Quickbooks error 3003:

1. Some synchronizations issues. 
2. Moving out of software, when sync is still running. 

What can be done to fix Quickbooks Error 3003? 
 
1. To fix Quickbooks error 3003, you have to close all the processes form the task manager. 
For this, go to end the processes tab in the task manager. 

2. Turn off the firewall and remove all the antivirus and security applications, if any.  If Quickbooks was already working with any security, then you have to get this updated.

Hopefully, Quickbooks error 3003 is now sorted!! For more help, you can reach out to Quickbooks enterprises support at +1-888-485-0289.