Powershell, 123 116 bytes

switch -r($args|% t*y){'\s'{$a=0}'\S'{$w+=!$a;$a=1}'(?s).'{$b++}'
'{$l++}}
 $c="$b".Length
"{0,$c} {1,$c} $b"-f$l,+$w

Powershell, 123 115 bytes

switch -r($args|% t*y){'\s'{$a=0}'\S'{$w+=!$a;$a=1}'(?s).'{$b++}'
'{$l++}}$c="$b".Length
"{0,$c} {1,$c} $b"-f$l,+$w

Powershell, 123 bytes

switch -r($args|% t*y){'\s'{$a=0}'\S'{$w+=!$a;$a=1}'(?s).'{$b++}'
'{$l++}}
$c="$b".Length
"{0,$c} {1,$c} {2,$c}"-f$l,+$w,$b

$f = {

switch -r($args|% t*y){    # evaluate all matched cases
    '\s'   {$a=0}          # any whitespace (newline not included)
    '\S'   {$w+=!$a;$a=1}  # any not-whitespace (newline not included)
    '(?s).'{$b++}          # any char (newline included!)
    '`n'   {$l++}          # new line char
}
$c="$b".Length
"{0,$c} {1,$c} {2,$c}"-f$l,+$w,$b


}

@(
    , ("a b c d`n", "1 4 8")
    , ("a b c d e f`n", " 1  6 12")
    , ("  a b c d e f  `n", " 1  6 16")
    , ("a`nb`nc`nd`n", "4 4 8")
    , ("a`n`n`nb`nc`nd`n", " 6  4 10")
    , ("abc123{}[]()...`n", " 1  1 16")
    , ("`n", "1 0 1")
    , ("   `n", "1 0 4")
    , ("`n`n`n`n`n", "5 0 5")
    , ("`n`n`na`nb`n", "5 2 7")
) | % {
    $s,$e = $_
    $r = &$f $s
    "$($e-eq$r): $r"
}

• $args|% t*y splits arument strings into chars
• switch -r($args|% t*y) evaluate all matched cases
  • '\s' case for any whitespace
  • '\S' case for any non-whitespace
  • '(?s).' case for any char (newline included)
  • '\n' case for newline char (newline represent itself)
• $c="$b".Length calculate a length of bytes number. $b is always max($l,$w,$b) by design
• "{0,$c} {1,$c} {2,$c}"-f$l,+$w,$b format numbers with same length. The variable $w converts to int. It need for strings without words. Other variables format 'as is' because $l and $b cannot be 0 because 'The input will always contain a trailing newline'.

Powershell, 123 116 bytes

switch -r($args|% t*y){'\s'{$a=0}'\S'{$w+=!$a;$a=1}'(?s).'{$b++}'
'{$l++}}
$c="$b".Length
"{0,$c} {1,$c} $b"-f$l,+$w

$f = {

switch -r($args|% t*y){    # evaluate all matched cases
    '\s'   {$a=0}          # any whitespace (newline not included)
    '\S'   {$w+=!$a;$a=1}  # any not-whitespace (newline not included)
    '(?s).'{$b++}          # any char (newline included!)
    '`n'   {$l++}          # new line char
}
$c="$b".Length
"{0,$c} {1,$c} $b"-f$l,+$w


}

@(
    , ("a b c d`n", "1 4 8")
    , ("a b c d e f`n", " 1  6 12")
    , ("  a b c d e f  `n", " 1  6 16")
    , ("a`nb`nc`nd`n", "4 4 8")
    , ("a`n`n`nb`nc`nd`n", " 6  4 10")
    , ("abc123{}[]()...`n", " 1  1 16")
    , ("`n", "1 0 1")
    , ("   `n", "1 0 4")
    , ("`n`n`n`n`n", "5 0 5")
    , ("`n`n`na`nb`n", "5 2 7")
) | % {
    $s,$e = $_
    $r = &$f $s
    "$($e-eq$r): $r"
}

• $args|% t*y splits arument strings into chars
• switch -r($args|% t*y) evaluate all matched cases
  • '\s' case for any whitespace
  • '\S' case for any non-whitespace
  • '(?s).' case for any char (newline included)
  • '\n' case for newline char (newline represent itself)
• $c="$b".Length calculate a length of bytes number. $b is always max($l,$w,$b) by design
• "{0,$c} {1,$c} $b"-f$l,+$w format numbers with same length. The variable $w converts to int. It need for strings without words. Other variables formats 'as is' because 'The input will always contain a trailing newline' and $l and $b cannot be 0.