@gulf.sabi, there is no way within QB to automate the combination of 3 separate QB files, but you can buy a third party utility.  Check out the first one at the link below.

http://q2q-us.com/q2qwiki/Utilities

Resolution for 'combination issue: 'Hello,Our Co.QuikBook Desktop is spitted in Three files(Year wise splitted) ,how we can upload all files in under one subscription?' available: Yes.

The combination issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the combination issue yourself by looking for a resolution described in the article. If it is a complex combination issue or you are unable to solve the issue, you may contact us by clicking here or by using other combination support options.

For more information about resolving issues related to combination in Quickbooks/QBO or to get support for the issue 'Hello,Our Co.QuikBook Desktop is spitted in Three files(Year wise splitted) ,how we can upload all files in under one subscription?', click the link below: