This is what i do to root my L3:
Rooting tools similiar to LG Spectrum, and take step-by-step guide following Dan Rosenberg Method. Here is the link: Root LG Spectrum Smartphone Method By Dan Rosenberg
Here is what his method:
Pre-Requisites to Root LG Spectrum
To Root your LG Spectrum these are few things you need .
1. Download the Root tool for Windows (http://vulnfactory.org/public/spectrum_root_windows.zip
), or the Linux/OSX version (http://vulnfactory.org/public/spectrum_root_linux_osx.zip
2. You have to Enable USB Debugging on your Device. To do this you have to go to development option in your appplication setting .
3. Make sure that your device has a charge of at least 60%, I would recommend you charge at least 90% to be safe .
How to Root LG Spectrum
1. Extract the tool which is in zip file you have downloaded .
2. You have to install LG ADB drivers if you are on Windows .
3. Connect your tablet via USB to computer .
4. Execute the “run.bat” file if you are windows or “run.sh” from a terminal (on Linux/OSX) . Wait till this get completed .
How to enable USB Debugging:
1. connect phone to pc
2. see usb icon on shortcut drop down, you see message : charge only , usb storage , go to pc-software and press ok.
3. go to settings menu- press applications - press development now you can press usb debugging press ok. USB debugging options should be able to tick
beforehand, you need to install L3 USB driver, find it from LG E400 Support: Find Manuals & Warranty Info | LG UK
click the software update & drivers
Hope it helps