The only way to unlock your Samsung Galaxy Tab 3 is to perform a factory reset. But, along with this process, you will lose all your installed applications. So, in order to perform the wipe process, first of all you need to shutdown your device. After that press the Volume Up + Power buttons and keep them pressed until the Samsung Galaxy Tab 3 logo appears. After that, use your volume buttons to select wipe data/factory reset and press Yes (using your home button). Reboot your phone after the process is ready.