Technically you're right - not more than two, maybe three, plants in each 15 cm pot, or they won't have room to develop properly. Oregano spreads sideways and not up - planted outdoors in a garden bed, it makes a good ground cover, spreading up to 2 feet easily in a sunny spot; it will cover the entire top of your pot over a fairly short time, so only one plant in there, though if you use it frequently, you could leave two to grow on on the strength you'll be cropping it a lot.
Now that the pots have so many plants growing in them, if they're still small, hopefully you can remove excess plants to leave just two or three in each pot without disrupting the roots of the ones you want to keep. It might have been better to just sow half a dozen seeds in each pot, because then it's easier to pull out weaker seedlings as they start to grow. If you can't remove plants without destroying them all, you might need to start again; hopefully you have some seeds left.
In response to your comment, you're right, supermarket potted herbs have more than one plant growing in the pot. This is done for speed, to get them ready for market quicker, and they only last a limited time if left uncut before they start to keel over.
If you're going to sow straight into your large pots, put half a dozen, spaced out, in each one, and allow 3 basil, 3 Coriander and two maximum Oregano to grow on, removing the rest. Unless, that is, you want to keep all six, cut them mercilessly as they get to a useable size, then re-sow from seed again in a couple of months, it's really down to how many leaves you need and how often you'll be using them.
The disparity between planting distances when grown outside is because, in that situation, the plants are given sufficient space to develop and grow properly, at both root and branch level, which means perennial ones (oregano for instance) can be used for cropping, but will survive year on year, and the annual ones (you'd sow more of those outdoors too if you use a lot) will carry on right through the growing season, without the need for frequent re-sowing.