On kingdom hearts 2 how do you get other summon characters besides stitch chicken little and genie?

You can only get Stitch, Chicken Little, and Genie as summon characters in Kingdom Hearts 2.

There are no other summon characters available in the game. The three you mentioned are the only ones you can obtain.

While there might be some misinformation online, there's no way to unlock additional summon characters beyond these three.