How do I report a problem in civicrm_api3 class?
The procedure for reporting bugs is detailed at https://civicrm.org/bug-reporting . This goes through verifying and reproducing the bug, and then it can be added on the issue tracker.

If you have a patch, you can submit a pull request on GitHub (often against https://github.com/civicrm/civicrm-core) with a reference to the bug report.