You cannot unlock the bootloader on the LG Optimus Zone 3.
However, you can root the device using a system-wide root strategy -- thus avoiding the inevitable risk of modifying the boot image, KingoRoot is the best method -- however, you must be running an older firmware build such as vs425pp2.
To downgrade to vs425pp2, you will need a KDZ or TOT firmware package and LGUP Dev Edition.
Once the root is established, you may use SuperSU-Me Pro to substitute the KingoRoot binaries with SuperSU root binaries. Then you can use FlashFire to upgrade back to a more recent firmware build, maintaining root with the EverRoot feature.