What I want to know is why 3PO thinks Lando and Chewbacca never returned from Jabba's palace?

Boushh then shows up with Chewbacca.

Take Luke out of it completely at first. Have him leave Dagobah and then go to Tatooine once Leia's plan goes to hell.

It would also give Jabba more weight as a villain if he screws up the heroes plans initially and Luke has to go in and rescue them.

Have the heores split up in the beginning. Luke is on Dagobah finishing his training. Leia and the rest are off rescuing Han.

But Jabba forces Luke to come to the rescue.

I dont know. Im making it up as I go. :p