ROCK/POP Roches, The: ''Keep on Doing.'' (Warner Records. 23725-1.) ''Keep on Doing'' is a charmingly rough-cut LP full of whimsical folk by three sisters (Maggie, Terre, Suzzy) who have no formal musical training. It's their third album, but it has the freshness of a garage jam session. Their voices have an honesty, an unengineered bluntness to them, that charms and at the same time seems thoroughly appropriate to their music. Their no-nonsense acoustic guitars back them capably. Minor defects aren't covered up - but this only enhances the feeling of immediacy. Most of the tunes are written by the Roche sisters themselves, like Terre Roche's tuneful, humorous, bittersweet ''The Largest Elizabeth in the World.'' ''Want Not Want Not'' can make it easy to clap away with an inanimate record. To the Roches: As the final cut on this album says, ''Keep on Doing What You Do.''