Flocks of 10 Guineas or more do better than smaller flocks, but if you don't have a lot of land for them then 6 would at least be better than 3. Hopefully you end up with more Hens than males, because that also helps keep the males occupied.
I free range my Guineas in my goat pastures (and with my horses and my donkey) and usually everybody minds their own business, but I have seen some extra cocky Guineas attack the goats before (and all my goats have horns, lol). Nothing major, and it doesn't happen a lot, but it just goes to show you what I mean about the male Guineas losing their minds once the hormones kick in, lol.
Also, if the enclosure you are keeping the Guineas in isn't covered, they will eventually fly out (and more than likely forget how to fly back in).
Having one big happy mixed flock would of course be ideal, but you may want to have separate coop and enclosure for the Guineas just in case they do become a problem. I keep a few Peafowl with one flock of my Guineas, and a few Turkeys with 2 other flocks of my Guineas (all of my flocks free range in separate areas/pastures)... the individual mixed flocks get along for the most part without any issues, but the Peafowl and Turkeys all are big enough to put the Guineas in their place if they need to.