It's actually "file://key.pem" when you want to give a relative path using unix systems. It will be three '/' in case of absolute path (e.g "file://home/username/..."). But this path consists of two '/' originated from "file://" and one '/' from the fact that home is a subfolder of the unix filesystem's root directory ("/home/username/..."). This two part will be concatenated and you will get three '/' characters following each other.
So you only have to concatenate "file://" with an existing path string in every case.