You can get a Smeargle in Black via a swarm on Route 5. In order to find out where swarms are, look at the big screen between routes.

I recommend going into some tall grass with Smeargle and a a pokemon with an attack that you need Smeargle to get. Eventually you'll get double battle, and when it happens have the pokemon with the needed move attack 1 of the wild pokemon and then have Smeargle target your other pokemon with sketch. That should give Smeargle the move.

Just repeat to get the other 2 moves.

Breed it with a Smeargle.

Yes, Smeargle is indeed the only way to get all of those moves. No other way.