Freddie here from the Shopify Support team, happy to help.
There are a few apps within the Apps store that can give you this feature, and here are the most popular:
Those are three of the most popular choices I saw, but you can find a full list of all the different apps that can work for this, right here.
In case you didn't know, it is actually possible to create these kinds of discounts using the Discounts section that is readily available in your Shopify Admin. It doesn't apply the discount automatically like these apps are able to do, but if you're looking for a completely free way to offer these discounts on your store, you can check out more information on how to set this up right here.
Let me know if this helps!
Shopify Support - email@example.com
Undoubtedly use a Quantity Discount app. It's compatible with any theme and optimized for mobile devices (iOS/Android). If you set up discounts for quantities of any products a customer buys, he will see the discounted price at the checkout.