I have always thought that the Prazipro instructions on the container are not clear regarding the 3 days. And I took it to mean the same as OP. You can redose after 3 days. Per this review of the product website "
https://www.aquarium-water-treatments.com/review/hikari-prazipro-review/" they say Prazipro will deactivate after 3 days. I would like to know what Hikari says about their wording but since it is a Saturday we can't contact them.
"Add PraziPro according to the dosage instructions on the bottle. Wait for 3 days for the Praziquantel to deactivate. Do a water change of at least 25% on day 4. Rinse and repeat process on a weekly basis to get rid of flukes. The follow up is important to eliminate freshly hatched fluke eggs. If your fish seems overly infected, treat tank with chelated copper, prior to the treatment."
Personally, I have not been successful using Prazipro to treat flukes. In desperation, I tried Seachem Cupramine. My understanding is it does contain copper but what other medicines, I don't know. If you look at the questions on their online instructions, it does state that it will treat flukes. It worked for me.