We were there last week, during a low-to-average crowd time.
Obviously, 3 FP+ isn't going to get you on every headliner in a park, so if you want to do all the most popular attractions, you'll have to combine the FP+ with rope drop arrivals (and/or evening EMH).
In our experience, the best use of your FP+ is to schedule attractions for mid-morning through evening that you otherwise can't get to after rope drop (e.g., if you're going to the MK and you plan to do Splash, BTMRR, POTC, Jungle Cruise and HM after rope drop, you can make your FP+ for the biggies on the other end of the park that you won't get to until later -- 3 choices from Space Mountain, Buzz, Peter Pan, Pooh, Dumbo, Mermaid, etc.)
Also, making those FP+ as soon as they become available for you is really important. If you want to change any of your initial three later, "on the fly," for the most popular attractions, be warned that this is really only works at the Magic Kingdom, which has enough headliners that it doesn't tend to "run out" of meaningful FP+. Whenever we attempted to make any changes to our FP+ for headliners (or first-tier attractions) in Epcot, HS or the AK, we found that those attractions had little or no availability. (For example, TSMM was totally "sold out" several days in advance, while Rock 'n' Rollercoaster only offered times late in the evening. Kilimanjaro Safaris was totally "sold out" at least a day before. Second-tier (or less popular, high-capacity) attractions like Tower of Terror, Star Tours, and Dinosaur were more readily available, and of course, we could always find FP+ for attractions where we didn't need it!
Same thing for the "additional FP+ after your first three are used" option -- to the extent it "works" (and it almost doesn't, because it's so inconvenient to have to go to a kiosk and wait in one line, so you can theoretically save yourself time in another line), only the Magic Kingdom had sufficient FP+ availability to make it worth our while.
We had more than our fair share of head-scratching moments, watching other guests try to make sense of FP+, like the people who showed up at Big Thunder Mountain Railroad immediately after rope drop and proudly scanned their MagicBands and entered the FP+ queue. (Meanwhile, standby was clearly displayed as 5 minutes and the attraction was a walk-on for everybody.)
Honestly, for me, the biggest change was having to readjust my idea of what a "popular" or "long line" attraction is. Thanks to FP+, many of the former "anytime" attractions (POTC, HM, Dinosaur, Aladdin's Magic Carpets, Great Movie Ride, Journey into Imagination) now have long enough wait times that they've actually become reasonable FP+ candidates during certain times of day.