Can I keep the number of submissions under 500 to keep using the FREE service?

We have info meetings and presentations on a monthly basis, but we do not need to keep their registrations for more than 3 months. If we deleted unneeded registrations does that allow us to keep using the free service? Plus we won't do over 100 registrations in one month. 

Yes.

Actually 500 is just the ceiling... you will be able to continue using your account and forms as FREE. 

The only downside is that older submissions will be remove from our server. So the submission data from oldest i.e. 501, 502... will be be deleted/removed.

 You can also just make a backup of all your data. User guide: How-to-export-all-of-your-data-at-once

And then delete older submission data to decrease the current submission counter (below 500).

User guide: https://www.jotform.com/help/377-How-to-Delete-Form-Submission-Data

Hope this is clearer. Let us know how we can be of further help.

Regards