Now obviously, these aren't creatures that are meant to be fought alone, as they would always surround themselves with those indebted to them. That being said, they are still a disappointing fight when they are meant to be these impressive creatures. For one, Wizards could've taken inspiration from the Baldur's Gate 3 Hag and give them the option to cast Mirror Image as a Bonus Action to underline their ability to fool others and be untraceable, or they could've invested more into their manipulative nature and give them the ability to cast Charm Monster.

what exactly is the threat posed by a coven of these? And what kind of deal can they make, since frankly anyone who needs to know obscure magic can probably just beat the hell out of a hag until she speaks. Not like the hag can do anything about it, and being in a coven just means more easy prey since they don't get anything even remotely useful from being in a coven anymore. Give them charm monster or at least charm person, misty step is a must, how do these things even escape since +3 stealth is not inspiring confidence that she can mask her footstep sounds even if she is invisible.

No bestow curse? Hags and covens without curses? WotC has become garbage.

I guess they wanted hags to be more of a Tier 1 foe...? It's the only way I can see the logic behind significantly weakening these iconic fey adversaries.

Escape

A hag can cast Invisibility to make an escape. If you prefer hags to have misty step or a similar ability, then have at it (I would recommend Fey Step as a bonus action rather than the misty step spell). But it is erroneous to say that the stat block itself is lacking escape options simply because you can think of a better tool than what the hag already has (invisibility).

Power

This stat block represents the average green hag. There are other kinds of hags, many of which are more deadly in physical combat. However, what makes hags dangerous isn't simply their ability to deal damage (reducing them to physical damage discounts the purpose of even using a hag—at that point, an ogre would suffice). Hags are dangerous because they are clever, deceitful, experienced, and surrounded by creatures they often bully into service.

There is also authority for making hags stronger than the average hag of their type. Auntie hags, for example, are generally stronger, older, wiser, and more experienced than the average hag. Grandmother hags are ever more impressive. Or, now, you could go all the way to Archhag. There's truly no reason to hark on the general green hag stat block for not posing a threat in your imagination (a CR 3 creature is meant to fought by low level characters). It's a little like going to a the Rust Monster and saying, "I can't believe my party won't struggle to kill this thing. What was Wizards thinking?!"

Hag Covens

In this edition, Coven Magic does not include damaging spells because the prior Coven Magic gave hags access to spells that dealt too much damage for their intended level of play. The result was that hag covens were too powerful for low level parties because of Coven Magic, but hags were too weak as monsters even with Coven Magic for higher level parties. This change allows hag covens to be antagonists for parties without requiring DMs to ignore Coven Magic (thereby making the trait a waste) or exposing lower level characters to spells that could often kill them outright.

Bestow Curse

I will infer your intentions here. The Bestow Curse likely does not offer what you are hoping for the hag to have at its disposal. I recommend utilizing the resources available to you as a DM to create a curse in line with what you are imagining and simply decide that the hag can inflict that curse on creatures (either by ritual or an action, dealer's choice). The DMG discusses curses here, including how to make them: Curses.

I don't like hags, especially these ones. 

New hag gained a swim speed of 30ft; Gained expertise in Arcana checks; No longer speaks Draconic, instead being swapped for Elvish. I don't have the Monster Manual 2014, so I have no idea how the Hag covens are changed. I'll edit this comment after I ask a buddy of mine who has the book. 

The hag used to only do one Claw attack, but now does two. Old claw did 13(2d8+4) slashing, new claws do 8(1d8+4) slashing plus 3(1d6) poison each. This raises the average damage from 13 to 22! 

The spellcasting is now Wisdom based instead of Charisma based, not like there's much difference between those two stats anyway. The hag gained Disguise Self in place of the Illusory Appearance trait, Invisibility in place of the Invisible Passage trait, and the level 3 Ray of Sickness. It lost Vicious Mockery. 

Fusion time! Bring back Draconic speaking, and bring back Vicious Mockery. Simple. 

I know some people just plug and play monster stat blocks but.....you don't need to stick with what's here. if your gonna use her as your big bad then you can add what ever spells you want to it. Hell an easy way of beefing up your human and humanoid monsters is to just flat out give them classes. 

To Paraphrase Captain Barbosa "The stat block is more what you'd call a guideline than actual rules" 

People are saying this is nerfed, but I still think it's a little powerful for CR3: they can cast ray of sickness as a level 3 spell at will. That's an average of 17 damage per attack. Like obviously these aren't statblocks for a boss, but that's still big damage for a CR 3.