I will start this with I am no power shell pro, only going by what I see in errors.
I managed to get the files missing, i also adjusted the script manually, in this one you do have the SurfacePro2 URL hard coded:
$DriverReleaseDate = "160128"
$DriverReleaseURL = "http://download.microsoft.com/download/2/0/7/2073C22F-2F31-4F4A-8059-E54C91C564A9/SurfacePro3_150623.zip"
$SourcePath = "C:\Source"
So even editing the $DriverReleaseDate it looks for the 150623 instead. So i edited
$file3 = $SourcePath+"\SurfacePro3_Win8x_160128_0.zip"
and hardcoded the file name
So now i got past that first part, things run and then i get a bunch more errors - does the script not create the folders it needs I think you had noted?
ERBOSE: Import processing finished.
import-mdtdriver : Path 'C:\Source\Drivers\Microsoft\Surface Pro 3\SurfacePro3 - March 2015\Drivers\Network\SurfaceGigabitEthernetAdapter' does not exist.
At C:\Source\Setup MDT 2013 for Microsoft Surface Pro 3 - June 2015-org.ps1:344 char:1
+ import-mdtdriver -path "DS001:\Out-of-Box Drivers\WinPE x64" -SourcePath "$Sourc ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : ObjectNotFound: (C:\Source\Drive...EthernetAdapter:String) [Import-MDTDriver], DeploymentPointException
+ FullyQualifiedErrorId : PathNotFound,Microsoft.BDD.PSSnapIn.ImportDriver
WARNING: Import terminated because one or more invalid paths specified