Do I need to create a W2c / W3c Correction if the retirement box is not checked on original W2 / W3 which were filed in January ? (S-corp contributed to owner SEP)
There's no deadline to file a corrected form, but the forms are required to be correct so you want to re-issue it as soon as you discover the penalty.
If it's only you as an employee, with the mistake, you may be able to get away with not correcting it since you can (and should) jus check the box on your personal return, when you enter your W-2 for your personal taxes. However, the fines are fairly hefty, so I would just recommend filing it.
Note, if you didn't check the box on your personal return, then additional penalties could apply if your retirement wasn't reported correctly.
It would be best to file the corrected form, but may not be necessary if it's only your W-2 because you know to account for it correctly. If this affects other employees, you definitely need to file a corrected version. There's no deadline to file a corrected form, but promptly after the error is discovered.
While it is required to file the corrected W-2, there may n your case, if it's only for your W-2 you just need to make sure you file correctly (which you can do since you are aware!) When you do your personal form, check this box so that the software can run your retirement plan contribution limits correctly.