"What will happen to R2D2 and C3PO at the end of ep. 3?"
well, whats important is not what happens in ep3, but what happens AFTER:
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after the great purge, 3po and R2 will be consumed by depression and guilt. their relationship will deteriorate forcing them to separate.
3p0 would go on to be a failed screen writer. after 3 failed attempts depression would fully take hold of him, and cause him to attempt suicide by jumping into sarlaac on tatooine....however, he was spit out since sarlaac had no taste for droids.
R2 took an even more self destructive path, sinking into a lifestyle of debauchery. sex and drugs would consume his soul and nearly end his life. R2's lowpoint was starring in an underground hardcore droid porno:
after 3p0's near-death experience, he saw that he needed to rethink his life.
he decided to seek out his old friend and help him on the road to recovery.
R2 later spent 3 months in the Coroscant Droid Rehab, and has been clean and sober ever since. upon leaving the treatment center, R2 met up with his counterpart, and they decided to tour the galaxy in the tantive IV and, as they put it, "find themselves".
and the rest is history......