ORA-00600/ORA-07445/ORA-03113 = Oracle bug => search on Metalink and/or call Oracle support
post excerpt from alert_SID.log file which shows all the errors which got reported for the most recent STARTUP
The error "ORA-03113: end-of-file on communication channel" means unexpected termination of your session.
Since you see the error during startup it's an instance failure, the cause could be crash of any BG process due to ORA-00600 or others.
Please check the alert log and provide the output.
One common cause for this is Oracle not being able to archive, as the FRA is full.
Try to see first whether startup mount succceeds, then consult the FRA.
Senior Oracle DBA