When you set ST3's Mixer channel outputs to 3+4 etc, the audio is sent to the corresponding outputs of the plug-in. To hear them you will need to set something up in your DAW host.
The method differs in every host (which one are you using?). In Reaper I just create all the output (audio) tracks for the plug-in and route the plug-in's output to them (this can be done with a few clicks.
Here is an example:
>>>
https://i.imgur.com/cPrbx05.png-- the Track Control Panels are to the left and the Track Mixer is at the bottom,
-- ST3 is on track 2 and its outputs are routed to the following 16 tracks (to channels 1+2 on each),
-- you can see (and I can hear!) the audio on tracks 3 to 6,
--I have set the ST3 track to show "multichannel metering"