BBC3 is really a sandbox for new programmes they would have only ever previously piloted on BBC2. Hence there is a lot of rubbish, but also a few real gems that now are mostly on BBC2. I don't know if it is a worthwhile use of the license fee or not, but some of my favourite comedy programmes in years have started out on BBC3. BBC4 is the Radio4 of TV, and I guess it has a very specific target audience, but the programming is generally good. Obviously both have a lot of repeats too, which most of the time makes them not worth watching.