Developer John Coates has just updated Flex 3 patching engine for iOS 11 firmware. You can now freely install it on your Apple device running iOS 11 firmware.

Along with iOS 11 support, you can now also use it on g0blin jailbreak for iOS 10.3.3. The beta version 43 also adds proper code signatures to existing frameworks.

Cydia

For the uninitiated, Flex is the ultimate hacking and tweaking tool for jailbroken devices. It contains tons of patches and hacks for various different iOS applications and games.

This allows you to get features that are missing from the stock versions of these apps.

You can download its Debian package and its associated dependency below.

Download Flex 3 (ios 11)  

Beta 43

RocketBootstrap 1.0.6~beta 1 (dependency)

Download the Deb file and extract.

SSH into your device and copy the /var/mobile/Library/Flex3/Flex.app folder and all its contents into the /Applications folder in the root of your iPhone. 

Copy the Flex.dylib and Flex.plist into /bootstrap/Library/SBInject

Go inside the Flex.app folder (current directory)

Type in chmod 777 Flex

Type in uicache

Source: reddit