How can I set up the looper to times signatures such as 3/4 or 7/8?
There is no time signature on the VoiceLive 3 looper.
So when you record a loop you decide of the time signature by recording a specific loop length based on your metronome (i.e. 12 beats = 4 bars of 3/4, 28 beats = 4 bars of 7/8 or 7/4).
Think of it as tape recorder: the advantage compared to a classic tape recorder, is that the Smart function will trim the endings to the beat (as you can use the metronome).