I used the encounter generator here, but I don't think it accounts for the Coven aspect. For my campaign, a Coven of Night Hags would make the most sense, but they're clearly the most powerful, so two sea hags and one night hag could also work.

So, basically, what level should a party of 6 be to fight A) a coven of 3 night hags, or B) a coven of 1 night hag and 2 sea hags?

Depends what else is going on. A level 5-6 party should be able to handle it.

I'll put this in a spoiler tag for those that don't have the sourcebook, but

Thank you both!

Hags are tricky. With monster statblocks in general, the statblock only tells what they can do in combat, and outside of combat, they can do whatever the DM thinks they should be able to do for the story. That goes double for hags. Volo's Guide to Monsters outright recommends inventing new powers for hags on the spot, and gives tips on how to do it. You're fighting a fairy tale monster, it doesn't need to be entirely logically consistent.

The key thing about a hag coven is that if you can focus-fire one of them the remainder is a lot less dangerous.

Ask yourself:

• Can my players take down a Hag in one turn? If so they break the Coven, and the Hags are pretty easy targets.
• Will the Hags be on separate Inititiatives?
• What happens if all 3 Hags win Initiative (more likely if you roll once for all of them) and all cast lightning bolt on that turn. Average damage 28, a character who makes 3 saves will still average 42 lightning damage. But the Hags can only do this whilst all 3 are still alive.