If you have already set the timeout to 3 hours, your best bet would be to download the Windows app for accessing QBO (rather than via a browser).

http://quickbooks.intuit.com/blog/the-quickbooks-windows-app-is-here/

Resolution for 'accessing QBO issue: 'How do I stop quickbooks from logging me out?' available: Yes.

The accessing QBO 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 accessing QBO issue yourself by looking for a resolution described in the article. If it is a complex accessing QBO issue or you are unable to solve the issue, you may contact us by clicking here or by using other accessing QBO support options.

For more information about resolving issues related to accessing QBO in Quickbooks/QBO or to get support for the issue 'How do I stop quickbooks from logging me out?', click the link below: