Your park hopper tickets are good for one day each. so you could chose one of your days to park hop.
I assume you are a family of 4, so each will have one park hopper ticket, once you link them to MDE. They will show up separately from the 3-day ticket.
The park hopper ticket is good for one day. If you then decide you want to park hop for another day, you can add the hopper option to the 3-day ticket. The price will be the same to add whether you do it on day 1 or day 3, you will still need to pay to add it for 3 days.