$Source = 'C:\Temp'
$NewFolderParents = 'A', 'B', 'C'
$NewFolder = '2018'
Get-ChildItem -Path $Source | Where-Object {$_.PsIsContainer} | ForEach-Object {
ForEach ($Parent In $NewFolderParents) {
If ((Test-Path -Path "$($_.FullName)\$($Parent)") -and (-not (Test-Path -Path "$($_.FullName)\$($Parent)\$($NewFolder)"))) {
New-Item -Path "$($_.FullName)\$($Parent)" -Name $NewFolder -ItemType Directory -WhatIf | Out-Null
}
}
}