Start with something very familiar, like a nursery rhyme or something like that. Most of those really basic nursery rhymes have a lot of really good musical theory behind them so don't shrug them off so quickly. Anyway, as you know the tune really well you can hum/whistle the main bit and then see how that relates to each of the 3 instruments you have. Like Superhuman said, try to record the basic melody on something so that you can play it back in a loop. With the 3 instruments you have, I would most likely play the bass track with the clarinet, the rhythm with the guitar and the melody with the piano. If you do use a sequencer to record the clarinet, you could even lower its pitch a smidgeon to really emphasise the bass frequencies of the instrument.

If you get stuck at any point, you can always cheat by finding the actual notes or chords for the song. Don't just use it to play the song though, analyse which chords are being played and why those chords work. Look at the notes in the chords, then look at the notes in the melody.. there will be a symmetry of the two.. as well as the bass line..

Anyway.. the point is to use a very familiar tune to start with.. like "bah bah black sheep" or something like that..