I want to buy Norton 3 years for 50 computers

If you could explain more about what you need that many licenses for, we can better guide you to the correct product.

Do you need the LifeLock credit monitoring service for all the devices you are looking to protect?

If this is for a business, there is a Norton for Small Business subsription that can cover up to 20 devices per subscription. Whatever Norton product you purchase, you are going to need multiple subscriptions.  And if you need central management, you might want to look into an enterprise level AV.