There are no leading zeros on the address.
If you reset your multifunction decoder, all the Configuration Variables will be restored to their factory default values, including the address. Any new locomotive will respond to address three (providing it has a decoder installed and no programming has been done). The same is true for a new multifunction decoder.
By specifying a default value of three in CV 1, anyone can acquire a decoder with factory default values and either operate the locomotive, or program it. This is necessary for a new decoder, or one that has been reset to factory values, so that the user doesn't have to sift through documentation for that decoder (providing you know who made it and what the product name/family is!) to figure out how to address it. Since most decoders are installed in a locomotive, this eliminates a lot of frustration.
It also allows the decoder with default values to be interrogated to determine who made it, and also read the decoder identification variable. Which may not be possible if you don't know the address.
After a decoder reset, the Primary Address is set to 3 in CV1. Any Extended Addresses have been reset and their CVs cleared.