Night hags have 8 outsider HD (with surprisingly useful class skills), as well as the Evil subtype (making them possible Fiends of X). Their ability scores are almost all above average, with strength the highest. They also have a huge amount of natural armor, as well as spell resistance, DR/cold iron and magic, and a few random immunities. Chassis wise, it seems they're pretty comparable to many other evil outsiders.
However, night hags are severely lacking when it comes to special abilities. Demon Fever is strong as far as diseases go, but still a disease, Dream Haunting is ridiculously weak, and the same goes for their SLAs (Sleep on a CR 9 monster, seriously?).
Change Shape into any small or medium humanoid is slightly less underwhelming, though on a chassis as great as the night hag's I'm wondering how often it'll be useful for anything other than disguise.
And than there's the heartstone. It's a minor magic item that requires a month to craft, gives immunity to disease and a +2 resistance bonus on saving throws (together worth about 13400 GP in magic items), as well as the ability to use CL 16 Etherealness at-will.
That. Is. Awesome. It allows you to bypass most, if not all barriers and monsters a mid-level party could reasonably expect, it makes for great spying, and the high CL ensures you can take up to five others with you. I'm not going to say it single-handedly saves the night hag, but it kind of does.
With this taken into account, I think +3 is an entirely reasonable LA. Discuss.