To the OP:

Everything Feeling" said is right on. Also..

If you're worried about the third band being a viable ensemble, don't.

That's not that ensemble's goal.

They need to work on fundamental playing skills and techniques. If they HAD those skills they wouldn't be in the third band. Use this as an opportunity to use the Book 2 of your preferred method book if you like.

If things go well, you can have them prepare the same music as the second band for UIL, too, and then you have a pool of fill-ins. And remember, no UIL rule prevents two ensembles from the same school from having the same concert program if it's within the performance requirements. If they REALLY come together and develop, just enter both groups!