You’re probably encountered some problems if you have upgraded to iOS 12 beta. Fortunately, users are allowed to downgrade iOS 12 beta to iOS 11.4 or iOS 11.3.1 in 3uTools as Apple still siging for it.

With the release of Houdini beta 3, jailbreakers could downgrade to iOS 11.3.1 to experience the latest jailbreak. 

You're suggested to back up your iDevice first, then you can customized restore it whenever you need.

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 you want to downgrade( All iOS on Easy Flash are available) as the following picture, finally click "Flash". 

Step 3: Keep a good connection between iDevice and PC, and then just wait patiently till this process is finished. The whole flashing process needs 5~10 minutes in general, when it's finished, then iOS11.3.1 should on your iPhone now.

Here is the walkthrough video for you.