There's what, a year between ESB and RotJ? Plenty of time for them to coordinate and come up with a broad rescue plan with plenty of redundancies--you can't put all your eggs in one basket when you deal with a bunch of bounty hunters.
These were never separate rescue plans--I got the impression that it was one big plan, and each had a part in it, and if one person failed, the others would be there to back up their buddies.
Obviously Lando was there for recon, but also as an extra gun when the time came, because there was no way Jabba was going to give up his prize carbonited Han without a big fight. It would have looked strange if everyone showed up all at once, so they had to sneak people in slowly.
Luke gave the droids to Jabba for backup and likely for recording purposes--they would remember everything said and everyone there--great info for taking down the Hutt's organization once they handed the relevant information over to some authority. Plus, giving perfectly good droids to Jabba virtually guaranteed that they wouldn't be destroyed, and would thus be available if Luke needed them during the rescue. Luke knew there was a high probability his weapons would get taken from him, including a lightsaber. With it hidden inside R2, he had an emergency back-up available.
Chewbacca was the excuse to get Leia in, and an extra pair of hands when the battle broke out.
They were going to need a number of people and droids to get Han out of Jabba's palace, and so these were just different parts of an overall plan to get all of them together.
The other answer: George wrote it that way, so that's why it shows up in the movie that way.