Here is complete guide how to Jailbreak iOS 11.3.1 via Electra Jailbreak Tool. It's first official jailbreak tool for all iOS 11 supported devices inlcuding new iPhone X, iPhone 8, iPhone 7, iPad Pro, iPad Air and other cool devices.
1) Download iOS 11.3.1 Jailbreak Tool here
2) Download Cydia Impactor on your computer and launch it.
3) Connect your iPhone, iPad, or iPod touch to your computer with an official Lightning cable.
4) Click and drag the Electra1131.ipa file over the Cydia Impactor window like so:
5) Sign in with your Apple ID when prompted:
6) Cydia Impactor will now begin installing the Electra jailbreak app on your device:
7) Once installed, go to Settings → General → Profiles & Device Management and tap on your Apple ID:
8) Enable "AirPlane Mode, Disable "Siri" and reboot your device
9) Next, tap on the blue Trust button, and then tap on the red Trust button inside the popup that appears:
10) After taking the recommended steps above to prepare your device, tap on the Electra icon from your Home screen and then tap on the Jailbreak button in the app:
11) After respringing, you should find Cydia on your Home screen. Done.