That is not without risk. It doesn't magically increase memory, but instead reallocates memory normally only used for internal processes. The 3GB switch can destabilize some things.
I did this on another computer, a windows XP, when Crusader Kings II failed to create save game files, making it pointless to play. After editing the boot.ini for the 3GB trick windows refused to boot properly, either getting stuck on an infinite CHKDSK loop or getting a "failed to load personal settings" error and kicking me out. It didn't even let me boot windows in safe mode. Fortunately, I had another computer which I could use to solve the problem. I could download Hiren's BootCD which would allow me to restore the boot.ini without actually booting windows.
I play PoE on another computer, a vista, so I might not get the same problem, but I'd rather not find out. I don't know if Hiren BootCD works vista, I think it's XP only. I don't want to take the risk.