You do have to choose
some kind of effect for each slot with the B3. However, the B3 retains whether or not you left slot 1,2 or 3 on or off the last time you used the patch. So essentially you could program anything into slots 1-3, but simply don't turn them on. Every time you dial up your "empty" patch, none of the effects in the patch will be active. This would send your signal through the patch without any effects. I don't believe the B3 is true bypass, but I will say that I am pretty hyper-sensitive to any tone degradation, and I notice nothing at all in the way of tone loss going through the B3 without effects being on. YMMV, as everyone's ears are different. I'd recommend ordering from any place that will offer a return policy if possible if you find the B3 degrades your tone in "bypass" mode.