I'm running it for 3. We're all new, though I've played some PF before, it's my first go as GM. So far I haven't modified anything, but I am planning to level them a bit faster and have fudged some rolls/rules to keep it fun for them. Technically, the GM is the players' guide not their enemy (XD), and you're the only one who will know if you're nerfing anything for their benefit. tl;dr: you can run it for 3 with minor mods I think.
