What does "submission receives 3 payments" means?

Is it 3 payments from each subscribers?

That's called payment submissions limit. When your form is integrated with any of the payment tools like Paypal, Stripe, the submissions will be counted as payment submissions. For the free plan, the payment submissions limit is 3 per month.

To understand more about the account limits, check this page https://www.jotform.com/help/408-Understanding-Your-Account-Usage-and-Limits