Hey there, J!
Welcome to the Disney Parks Moms Panel. Your trip is right around the corner, and I can't wait for you and your family to make magical memories at the Happiest Place on Earth. As a mother of a three year old boy as well, I'd love to give you some advice when it comes to riding attractions at the Disneyland Resort.
As far as thrills go, I will say that
Guardians of the Galaxy - Mission: BREAKOUT! is possibly one of the most intense attractions at the Disneyland Resort. This ride features sudden drops as well as periods of darkness and some possibly scary visuals. I would say that I'd want to be very sure my son would be OK with these experiences before possibly putting him in a situation that would make him uncomfortable. My best advice is to work your way up starting with some of the less thrilling attractions, and see how he reacts. My son's first coaster was Gadget's Go Coaster in Disneyland Park. This fun little roller coaster lets little ones try out some small turns and tiny drops. My son LOVED it, but I also have a 5 year old niece who thought it was too scary. Every child is truly different. If your son has a blast and wants more thrills, next I would suggest trying Big Thunder Mountain Railroad, Radiator Springs Racers, Splash Mountain, and maybe even Space Mountain. To me, all of these are less intense than Guardians of the Galaxy - Mission: BREAKOUT! so if he starts to get upset with these, I would say to not attempt it. You can always try more on your next visit!
There are still plenty of ways to have an amazing time even without thrills like meeting favorite Characters, watching unique entertainment, and enjoying some tasty treats. If you have any other questions, please feel free to return. Have a Fantasmic day!
-Jennifer