Are they though? The stories of the three sisters/three muses are the material upon which thes particular monster is built, similar stories about solitary crones may have overlaps but do they need to be the same creature?
Why ask if there are stories about solitary hags that curse when you acknowledge they exist in the second half of your sentence? The question is not whether the Coven also exists - it is whether there are folklore origins that are the solitary hag, and we both agree there are.
I’m all for creating unique solitary monsters that have their own abilities - if you want Baba Yaga as a minor fae goddess in Hag form with unique strengths, curses and a flying pestle, go for it - but thats different to discussing the Hag in the Monster Manual.
And the Hags discussed in the monster manual lack key features common to their folklore origins, the Hags in the monster manual differ from the common Hag of folklore. That is the issue we're raising.
Putting it another way. Let's say they add a monster to the monster manual. It has four fairy legs, and two red eyes. It has claws that it brutally uses to slash people with and it can use those legs to trip opponents 10 feet away. They call it a goblin. This is not the goblin we expect. This is not what we think of when we think of goblins. It works fine as a monster, but it just doesn't evoke what we expect. That is the problem here - the Hag, as evidenced clearly and without room for reasonable argument in Baldur's Gate III and Critical Role's Campaign 2, is expected to have the power to elicit curses and supernatural styles of magic based upon their folklore representations. The versions we've been provided have this deficit. It is far from a tragedy, but it is a disappointment they could have done better.
A hag, literally, is a folklore witch. That is where the name originated - it comes from a word that meant witch The difference between them and a witch in most lore, however, is that a witch was once human while a hag was always something else.
If I'd been involved in 5E construction, the Hags would start at CR 7 and go through CR 19. They'd have a variety of cursing powers, but these would not be their combat powers, generally. Instead, the curses would be used as plot devices and challenges. They'd be intended to be one of the iconic challenges of a campaign or adventure, with a setting placed around them that turns on their existence, much like the Realms of the Ravenloft setting were designed around the Lord of the Realm. In my opinion, if you have a Hag, it should be something that provokes Folklore in the region. Otherwise, it doesn't bring anything iconic to the table and it just becomes another layer of a boring sandwich.