Locked Bootloader of Verizon Galaxy S3 always a discouraging steps towards development. However, noting manage to set back the development process and now developer manage to develop an one click app [VZW GS3 EZ-Unlock (Bootloader)] that can easily unlock and lock your Verizon Galaxy S3 SCH-I535 by pushing a button. Notably the app don’t require you to allow app installation from unknown source but this is a Google Play store app.
What you require now is, a rooted Verizon Galaxy S3 to make this app work. You can find Verizon Galaxy S3 rooting method here – How to Root Verizon Galaxy S3 & Install ClockworkMod Recovery.
Google Play link for VZW GS3 EZ-Unlock (Bootloader)