c:dir1dir2dir3file.txt

PS> Split-Path (Split-Path c:dir1dir2dir3file.txt -Parent) -Leaf
dir3

PS> ([uri]"c:dir1dir2dir3file.txt").segments[-2].trim('/')
dir3

PS> (dir c:dir1dir2dir3file.txt).directory.name

(Get-Item c:dir1dir2dir3file.txt).Directory.Name

$path = [System.IO.Path];
$path::GetFileName($path::GetDirectoryName("c:dir1dir2dir3file.txt"))

"c:dir1dir2dir3file.txt".split("")[-2]