Welcome to planDisney, Emma!
How exciting that you're planning a 3-day trip to The Happiest Place on Earth. I bet the anticipation is building day by day. Having recently returned from the Disneyland Resort, I can tell you that you'll have a fantastic time!
Like you, since I was going for multiple days, I decided not to get the Park Hopper ticket in order to save some pennies. I didn't regret that decision because each park had more than enough to keep me entertained for the entire day. You'll be pleased to know that you don't necessarily need a Park Hopper ticket in order to visit Star Wars: Galaxy's Edge and Avengers Campus. Although you wouldn't be able to visit them on the same day. Let me explain why:
Star Wars: Galaxy's Edge is one of the wonderful lands located in
Disneyland Park. Therefore, if you make a park reservation for Disneyland Park, you'll be able to visit Star Wars: Galaxy's Edge during that day. Make sure you ride
Star Wars: Rise of the Resistance and
Millenium Falcon: Smuggler's Run!
Avenger's Campus is located in Disney California Adventure Park (right next to
Cars Land). Therefore, if you make a reservation for
Disney California Adventure Park, you'll be able to visit Avengers Campus during that day. Here's a tip: arrive for park opening and head straight for
WEB SLINGERS: A Spider-Man Adventure. My family had so much fun on this attraction! Although our arms were exhausted by the end. It's tough being a superhero!
I hope that's helpful, Emma. Have a wonderful time on your Disney adventure.
Mike