Apple has stopped signing iOS 11.3.1 after releasing iOS 11.4 update. This mean you can no longer update or downgrade to iOS 11.3.1.
Previously, we suggested you to downgrade to iOS 11.3.1 as a working Jailbreak is coming very soon. If you missed to downgrade then you have missed a chance to Jailbreak your iOS device. If you have saved shsh blob of iOS 11.3.1 then you can still downgrade using futurerestore tool.
Coolstar has already started to build up stable Jailbreak tool for iOS 11.3.1 as Ian Beer has released Kernal Exploit for that iOS.
Be prepared to Jailbreak your iPhone, iPad and iPod Touch as it might be coming any time soon!