Apple is releasing its third iOS 12.1.3 developer beta, after releasing its second iOS 12.1.3 beta late last month. The release notes only mention bug fixes.
Step 1 Connect your iDevice to PC with USB cable and launch 3uTools.
Step 2 Navigate to Flash & JB → Easy Flash and select the firmware iOS 12.1.3beta 3. There will be a popup after you click Flash, please read it carefully. If you want to save your data, please click the checkbox "Retain User's Data While Flashing".
Please back up your iDevice regularly just in case.
Step 3 Keep a good connection and internet while flashing, wait patiently for around 5 to 10 minutes. Your device will be upgraded to iOS 12.1.3 beta 3 after reboot.