Damir
Damir the master EQ should only be required to correct for change in PA equipment
This should be done on the master EQ on your mixer, not in ST3 so that your vocals and any instruments are also corrected for the speakers / room. I always run a graphic EQ at the end of my mix and only buy mixers with a master graphic. That's quite difficult to do nowadays when you're trying to get the smallest mixer possible. They also provide you with fewer frequencies on the small mixers, but you can make most things right on 7 or more bands. I definitely avoid 5-band "master" graphics EQs. I currently use a MACKIE ProFX8 V2 which does a fair job with only 7-bands.
I personally don't use any EQs in ST3; only a 6dB boost to the master gain. If, however, I was acquiring ready-made backtracks then I'd probably be EQIng them per song. The only use case for the ST3 master EQ for me is when I'm running backtracks to something like a portable bluetooth speaker, while singing and playing guitar unplugged...
Damir But you have to have some reference point
Yes, that's the first bit of advice I'd give someone new to setting sound. Find a well-balanced mix, learn what it sounds like, and use that from now on to set your master EQ. Once you have that mix sounding good, then work backwards with minor tweaks on your channels or per song which then should not change from gig to gig.
I use Mark Knopfler's song "What It Is" when listening to studio headphones and live PAs in-store. Why that song? Not because it has the best mix ever, but because I'm happy to sound like how I can make that song sound. Live, I play one of my own tracks that isn't compressed, and has more punch, which was mixed in studio with monitors that sound great playing "What It Is". Setting master EQ at gigs now takes less than 1-minute, and I have no need to adjust any channel EQs... But to get down to 1-minute might take someone 15-years of experience to achieve. Start by learning what each slider sounds like, also so much easier to do on a graphic than on a parametric.