Rare instrumentation?

Arrangement is going to be your best bet.

Find the piece you all are going to perform and work out where doubling organ pedal and tuba will work (or have the Tuba carry all of that weight if your organist is a "Left foot Lucy"

Have the clarinet play melody at strategic places or, in the case of hymns, the tenor line is often interesting.

Do what you can to keep the music "fresh"; that is don't do Clarinet melody for the entire piece and Tuba only for the bass line. Do some interactive parts, call-and-response, and or echo (same line with off-set entry).

you're going to be better off making your own arrangement. Luckily, the tuba and organ are both in the Key of C (which means that the note they play is the same note that sounds) The clarinet is in the key of Bb, which means that whatever note it plays, the note that sounds is a whole step lower. So, if you transpose the music up one whole step, it'll be in the right key.

Sorry I can't be of more help, but in all my years, I've never seen a clarinet/tuba/organ trio. Hope it goes well.