A free account gets 3 reports, ever — not 3 a month. That is enough to research a real idea and the two obvious alternatives, which is the point at which you know whether this is worth paying for.
After that there are two ways to carry on. A pack is 10 more reports for $9, bought once, and they do not expire. Pro is 150 reports a month, counted against your billing period, with a ceiling of 20 in any 24 hours.
Reading a report you already have never counts. Only starting a new one does.
Pending and running reports reserve an allowance or purchased credit. Completed reports consume it; a failed report releases it. This is not a refund of a pack purchase.
Every account is limited to 20 new report attempts in a rolling 24 hours, including failed attempts and purchased-credit reports. Hitting this limit does not spend a credit.