From the StoreApp start page, Click on Administrative, and from the drop down list, click Products and then click on Price Levels
On the left hand side, click on the product you want to set a Price Level for, on the right hand side, click Add Price Level
On the Bulk Price Dialog, enter the following information:
Click Submit to complete the process.
The product price will automatically change once the item purchased is three and above, The product will reflect like this once the process has completed.