If we accept all the Star Wars films as the same canon, then a lot that happens in the original films has to be reinterpreted in the light of the prequels. As we now know, the rebel Alliance was founded by Yoda, Obi-Wan Kenobi and Bail Organa. What can readily be deduced is that their first recruit, who soon became their top field agent, was R2-D2.
Consider: at the end of RotS, Bail Organan orders 3PO's memory wiped but not R2's. He wouldn't make the distinction casually. Both droids know that Yoda and Obi-Wan are alive and are plotting sedition with the Senator from Alderaan. They know that Amidala survived long enough to have twins and could easily deduce where they went. However, R2 must make an impassioned speech to the effect that he is far more use to them with his mind intact: he has observed Palpatine and Anakin at close quarters for many years, knows much that is useful and is one of the galaxy's top experts at hacking into other people's systems. Also he can lie through his teeth with a straight face. Organa, in immediate need of espionage resources, agrees.
For the next 20 years, as far as 3PO knows, he is the property of Captain Antilles, doing protocol duties on a diplomatic transport. He is vaguely aware of the existence of the princess but doesn't know much about her. Wherever 3PO goes, being as loud and obvious as he always is, his unobtrusive little counterpart goes with him. 3PO is R2's front man. Wherever they land, R2 is passing messages between rebel sympathisers and sizing up governments as potential rebel recruits - both by personal contact and by hacking into their networks. He passes his recommendations on to Organa.
Yoda is out of the picture by this stage, using the Force-infused swamps of Dagobah to hide himself from Vader and the Emperor. Or something. He is meditating on the future and keeping in touch with Obi-Wan via the ghost of Qui-Gon Jin, which as comm systems go has the virtue of being untappable. Obi-Wan, on Tattoine, keeps in touch with Bail Organa and the other Rebel leaders by courier, of which more later.
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