iOS 12.1 beta 3 comes one week after Apple seeded the last beta build to developers. The company also released the third beta builds of watchOS 5.1 and tvOS 12.1 to developers today. If you want to upgrade to this version, just download and flash in 3uTools.

Please back up your iDevice regularly just in case. 

Step 1: Connect your iDevice to PC using the USB cable and launch 3uTools on your PC. 

Step 2: Click "Flash & JB" -> "Easy Flash", then select the firmware iOS 12.1 beta 3 as the following picture, finally click "Flash". 

If you want to save iDevice's data, you need to select the checkbox that "Retain User's Data While Flashing"

Step 3: Keep a good connection between iDevice and PC, and then just wait patiently till this process is finished. It would take 5~10 minutes in general. After that, you can feel free to experience iOS 12.1 Beta 3.