Don't Install iOS 11.3 If You're Jailbroken
If you want to jailbreak your device, avoid the iOS 11.3 update.
Jailbreak developers recently released a working iOS 11 jailbreak for iOS 11.0-iOS 11.1.2, but they haven't released anything for iOS 11.2 and above.
The iOS 11 jailbreak is for advanced users and it doesn't come with Cydia. At least not yet. Novice iOS users should avoid the iOS 11 jailbreak for now.
Unfortunately, there's no way to downgrade to an older version of iOS 11. If you're running iOS 11.3, iOS 11.2.6, iOS 11.2.5, iOS 11.2.2, iOS 11.2.1, or iOS 11.2 right now, you're stuck until developers release something for those updates.
Keep an eye out for new developments as we push on into 2018.