What we're trying to accomplish is using a high-gain guitar amplifier in conjunction with VoiceLive 3's guitar effects, and the tricky part is we also want to guide vocal harmonies with the guitar signal.  If you typically play clean guitar this wont really apply to you, this is intended to be used with a distorted guitar.  The issue is that VL3s harmony algorithm is expecting a clean guitar signal for guidance, so that it can actually hear the notes you're playing.  This means we can't send the VL3's guitar output direct into the amp (we don't want to distort reverb or delay effects) and we can't send the amp's output into the VL3 (harmonies won't hear your distorted chords).  So the finicky solution is to use a splitter and the amp's FX loop.

Yes, we realize this wouldn't be a problem in the first place if the VL3 had an effects loop, when VL4 releases in ten years it will hopefully rectify this ;)

We need to split the guitar into two clean signals, send one copy of the guitar to the amp, and another to the VL3 for harmony guiding.  Here are the two guitar paths:

Path 1 Audible Affected Guitar:This will be what you and the audience hear.  Your guitar will be distorted by your amplifier, and will have any additional effects from the VL3 youre using.  It will not be used for guiding the harmonies because the signal will be distorted by your amp.Guitar -> Splitter -> Amp Guitar Input -> FX Loop Send -> VL3 Guitar Input -> VL3 Guitar Output -> FX Loop ReturnAs you can see were basically using the VL3 as a pedalboard insert for the amp.  This way you can use your amps distortion to your hearts content and still apply clean reverbs, delays, choruses, and whatever other effects overtop of it.  Youll need to go into the Setup menu, and on the Guitar tab turn off the Speaker Simulation we dont want this because youre using the speakers in your amp.User-added imagePath 2 - Silent Harmony Guiding:
This wont be heard by anyone but our harmony algorithm.  Its basically a clean and silent guide so the VL3 knows what chords youre playing.
Guitar -> Splitter -> VL3 Auxiliary InputYoull likely need a 6.5mm to 3.5mm adapter to get from the splitter into the Aux in on the VL3.  The physical setup is simple, but there are a few settings in the VL3 youll have to set to get this working right.In the Harmony effects editing page, scroll down to NaturalPlay Source and set it to Aux/Mon.  This tells the harmonies to ignore the signal from the Guitar input, and instead only listen to the Aux input which is the clean version of what youre playing.  Youll have to set this in every preset you use with this setup.User-added imageIn the Mix menu, on the Main Mix tab youll need to turn the Aux level all the way down to Off.User-added imageI did the same on the Headphone tab because I was using phones to monitor myself. User-added image That's about it!  Here is the setup on paper:User-added image...And here it is in practice:User-added imageUser-added imageIn my setup I'm using an A/B pedal with both outputs active to split the guitar signal.  You could also use a 6.5mm Y-cable to accomplish the same thing.
Keep in mind that your playstyle will still affect how effective this is.  Our harmonies need to be able to hear at least the root and third of the chords you're playing, so if you're soloing or playing monophonic melody lines, the harmonies will likely sound wrong.