The later in August the better, but honestly, I’ve never found Express Pass worth it. On the busiest days of the years, it might be worth splurging and staying at a deluxe (but still not buying it separately), but you should be absolutely fine if you’re spending 3 days at Universal.
Universal Studios is easy to knock out in a day with a good mix of shows to pad the busiest times of the day.
I’m not sure the age of your party, but the best rides are Mummy, Transformers, Gringotts, Men in Black, ET, Simpsons, and Rip Ride Rocket. ET and Men in Black don’t have bad waits, generally.
Men in Black, Transformers, Rip Ride, and the god awful Fast and Furious ride all have single-rider lines that I’ve found drastically cut wait-time.
I’ve had mixed results with Gringott’s single rider, but it also does bypass cool parts of the queue, so probably better to stick to the normal line.
Make sure to do the Bourne Stuntacular (I’ve found sitting close to the back makes the experience better), as it’s exceptional. Best to do it mid-day (like 2:00 pm) when wait times are highest.
I’ve found rope dropping the park can easily get you on Rip Ride, Mummy, Transformers, Men in Black, and ET all in a very compressed amount of time. Then doing Simpsons might be a good option, but the pre-shows make it take a lot longer. At this point, you’ll only be a couple hours into your day, and you can choose to do one of the mediocre rides (Fallon, Fast and Furious, or Despicable Me), grab food, explore diagon alley (fantastic land and absolutely worth exploring and just spending time in), or one of the shows. Animal actors is cute, Bourne is elite.
Eventually either bite the bullet for Gringotts or wait until waits die down near the end of the day.
The night show at Universal Studios (not shown every night) is shockingly good, but it’s not shown every night, but it’s worth doing.
You really should have no trouble doing US in a day without express pass, except on the busiest of days (and then just skip the mediocrity).
As for Islands, islands of adventure is a world class theme park that’s absolutely fantastic if you have park hoppers, the bulk of your time will likely be spent here, if you don’t, spend two days here.
In the summer when thunderstorms are ever abundant, due to the high percentage of out-door attractions, I recommend doing them as early as possible in the day (if there’s consistent rain forecasted for a day, go to universal studios).
Save Forbidden Journey, Spiderman, Kong, and Cat in the Hat for later in the day, because they are literally the only indoor rides, and everything else will close. Saving these rides isn’t as important since you have multiple days, but regardless.
Hulk, Dr. Doom, VelociCoaster, Popeyes, Dudley Doo, Jurassic Park, Hagrids, Flight of the Hippogriff, etc, all close during lightning, so the park basically shuts down.
VelociCoaster, Hagrids, Forbidden Journey, and Spider-Man are absolute must rides that should be experienced at all costs.
Dr. Doom is pretty trash, only do it if you’re walking by and it has no wait. I’ve personally only ever ridden it once, and it was immediately after it reopened following lightning so there was no wait. Storm Force accelerator is a basic flat ride, and the Cat-in-the-hat attractions can be skipped unless specific interest or demographics from your party specify it.
Flight of Hippogriff is a basic kids coaster.
Kong, Dudley, Popeyes, Hulk, and Jurassic Park are all worth doing.
My strategy and recommendation is rope dropping the park, immediately going left and riding Dudley, then Popeyes, then checking the wait time for Hagrids and VelociCoaster. If the wait for either seems reasonable, go to one of them. If not, do Jurassic Park River Adventure. VelociCoaster’s wait seems to fall quite early in the afternoon, plus single-rider offers a great way to shorten the wait, so prioritize a low Hagrid’s wait over VelociCoaster.
I might honestly recommend rope-dropping Hagrids then VelociCoaster on your first day at Islands, just so you can experience them early and determine how you want to spend time re-riding them.
I 100% recommended doing Hagrids before VelociCoaster because Hagrids feels too tame after riding VelociCoaster.
I’m going to stop rambling, but moral of the story, don’t waste your money on express pass