If you wish to hard reset the Sony SGP621CE Xperia Z3 Tablet Compact LTE then there are only two methods. The first is by software such as the Xperia Companion which also allows you to backup your settings and data before the reset. The other method is by using the menu.
Hard Reset Sony SGP621CE Xperia Z3 Tablet
Unlike the Sony Xperia P LT22i Nypon or the Sony Xperia TIPO dual there are not code resets on this Xperia model. The SGP621CE Xperia Z3 Tablet reset does not allow for a factory reset by code. Much like other mobile phones in the Xperia range. Once you have reset the phone the data and settings cannot be recovered. Most people tend to reset their phone when they want to sell the mobile phone and delete all their personal data.
Hard Reset will erase all of your data
All described operations you are doing at your own risk.
Hard Reset Sony Xperia SGP621CE Xperia Z3 Tablet