no longer than 3 hours of the event ending.
This isn't grammatically correct, and should be modified. Here is one option:
no later than 3 hours after the event ends.
Tickets must be submitted
should most likely be
Tickets will be collected
This change isn't about grammar, but about choosing idiomatic words to go with "events" and "tickets".
Finally, even with these changes the whole sentence still doesn't make sense. You don't go to an event and submit (or collect) a ticket - three hours after the event has ended. It's grammatical but nonsensical like "I turned off the television, to watch the program."
Unless you don't mean "ticket", but perhaps "voucher for a free meal", or something else.
By the way, it is possible to use "of" (which appeared in your original sentence) when discussing a time period. However, it doesn't go with "no longer".
If you get to the hospital within 3 hours of the first symptoms of an ischemic stroke, you may get a type of medicine called a thrombolytic.