A cron task works well for resetting a value like this one, and caching a value like this one is a reasonable approach to solving this problem. But, keep in mind that you're caching this value, and cache invalidation is a hard problem that can often lead to bugs & additional complexity.

counting posts

Rather than caching, my first instinct would be to count the number of posts each time. Here's some pseudo code:

const count = await posts.count({userId, createdAt: {$gte: startOfDay}});
// alternative: const count = await posts.count({userId, _id: {$gte: startofDayConvertedToMongoId});
if (count > 3) throw new Error("no more posts for you");
await posts.create(newPost)

(note: if you're worried about race conditions, any solution you choose will need to check the count in a transaction)

If you have an index that starts with {userId: 1, createdAt: 1}, or if you use the _id instead {userId: 1, _id: 1} (assuming that you're not allowing client _id creation), these queries will be quite cheap, and it'll be hard for them to get out of sync.

separate cache collection

Even if you do decide to cache these creation values, I'd recommend caching them away from your user's collection to keep your collections more focused. You could create a post_count collection and then update only the cache collection for these counts: post_count.updateOne({userId, day}, {$incr: {count: 1}, $setOnInsert: {day, userId, count: 0}}, {upsert: true});. One nice benefit of this approach is you can use a ttl index on day to have mongo automatically remove the old documents in this collection after they've expired.

Since you are using MongoDB I would suggest,

1. Use agenda and create a job that runs at UTC 00:00 (If you have diverse users from different time zone) or time zone specific to your user's country.
2. In this job call updateMany function on your user model to reset dailyPostLimit field.