tu tu ku, ta ta ka, or to to ku, depending on the range you are playing in. Do what's comfortable, but start slow, practice cleanly then increase your speed as you go.
For real hard-core practice do some k-tongue exercises like, ka ka ka - ka ka ka - ka ka ka.
This will help develop ka tongue which will eventually improve your triple-tongue speed.
Also, make sure to keep your air flowing and think of your tongue as just a way of interrupting that air supply. Don't close off your air to make the sound.