As @PrincipalTinker, @Twynyrodyn and @rebeecky have all have said, UOR is very easy to tour with Expresses Passes. You’ll have no trouble visiting all the attraction in 3 days.
Here is my additional advice from my June 2017 UOR trip as a newbie.
I went in June 2017 with my parents (in their 60s), 23 yo younger sister and significant other (we’re in out early 30s). My family had early entry and onsite express passes, but insisted on having a plan as well (which was pretty much ignored in the park 
). However, the planning process was helpful to learn about the parks.
I used 2 different touring plans for our park time from Touring Plans (when we were at UOR the HP headliners did not have express access. They do have express access now.)
Day 1 - Wizarding World of Harry Potter Comprehensive Touring Plan for our first day (We are big HP fans).
Day 2 - 2-Day 2 Park Plan with Early Entry - Day 1
Day 3 -2-Day 2 Park Plan with Early Entry - Day 2
One important thing about UOR is that they have mandatory ride lockers on several thrill rides. These lockers are free to use for the duration of the standby wait time and ride. Additionally, they enforce a “no loose items” policy on the big coasters (Hulk and Rip Rockit); riders need to pass through a metal detector prior to boarding.
In addition to Touring Plans, Orlando Informer is another unofficial planning website that is very helpful for UOR information. I really appreciated OI’s photos of things like turnstiles and ride lockers as I really had no clue about the park layout prior to visiting.
OI has a 3 day park-to-park touring plan for onsite guests
If dining selection is important to you, here is a thread on Universal Dining.
Also adding that the Harry Potter areas in both parks get physically congested even when overall crowds are low. To explore Hogsmeade and Diagon Alley with lighter crowds, visit during Early Entry or in the last hour of park operation if possible.