Step 1:
From the StoreApp start page, Click on Administrative, and from the drop down list, click Products and then click on Price Levels
Step 2:
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
Step 3:
On the Bulk Price Dialog, enter the following information:
Step 4:
Click Submit to complete the process.
Step 5:
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.