I have often gone in pairs of 3’s and it has worked out fine, they actually have single rider lines so another person would be sitting in that lone seat. You can make new friends!
The majority of the rides allow more than 2 people sitting together, but there are exceptions. Here are some of the major attractions:
Indiana Jones: Large car with 3 rows of up to 4 people.
Thunder Mountain: cars on train with 2 per row.
Matterhorn: 3-4 people per sled
Space Mountain: cars on train with 2 per row.
Splash Mountain: logs with 4-6 per log, single file
Star Tours: Large Theater, 8-10 per row.
Autopia: 1 or 2 per car.
Mad Tea Party: 2-4 per teacup
Haunted Mansion: 2-3 per Doom Buggy
Jungle Cruise: 20 per boat
Small World: boat, 2-4 per row
Pirates of the Caribbean: boat, 2-5 per row
Buzz Lightyear’s Astro Blasters: 2 per car
Family/Kid rides, like the Dumbo, Mr. Toad’s Wild Ride, Snow White’s Adventure, etc, have 2-3 per car depending on size.
California Adventure is similar: Some rides take 2 per car, but most take more.
To be honest, I think that you will have a lot of fun even with 3. Yes, you might have a couple rides where 1 person is alone in a seat or with a “stranger”, but I just use it as an opportunity to make a new friend. I figure that we are all there enjoying the park, so meeting someone new is fun. I always like to find out where people are from and where they are staying. I definitely wouldn’t let it stop me from going.