Hello maxprinciotta,
This is a great idea, but would create an extra layer of validation that would be more than a small bit of code. Unfortunately that level of customization goes beyond the scope of the forums.
If you have a moment, we’d love for you to submit your idea to our UserVoice forum. We take user ideas and contributions very seriously, and many times their ideas end up in our future releases.
Out of the box without adding any code, the best way to do this is have users submit drafts and have an admin screen for large events.
Additionally, adding a message at the top of the page (making it as bold as you care too) may limit the amount of large events you receive, as users self screen.
You could use some front end validation to do some date math with jQuery. The answer to the problem is inherently straight forward. Set a click event on both date fields, and screen to make sure the events are less than 3 days apart, notifying the user if they are otherwise. This would be the fastest, but there may be other forms of validation (back end support) that would create a more robust solution.
Hope this helps! Let me know if I can offer any more pointers.
Cheers!
– Casey Driscoll