if you haven't figured it out by now (and also just in case anyone else who also had trouble with this needs it), I believe this is what you have to do: make sure Timber "kisses" you (as in licks your face) - to do this, I just selected both flirty options on the witch route - and then let him shower (alone). after that, choose to look at him when he comes out of the shower. that should unlock a fourth shower CG that's similar to the first three but with a different expression.