Hi, Victor, the proximity sensor is connected with the ear phone, which means you have to replace both of them if you want to replace the sensor. You can watch this to find the detailed position (at the last several steps) and learn how to replace it.
You'd better get the replacement screen for your Verizon version for fear of causing any trouble. And the new screen with front housing for S3 is not cheap. The S3 model for Verizon is SCH-I535.