Comparison:

Why use KM3 when I could just use Aegisub automation?

Although Aegisub automation has some advanced features that km3 does not, it is way more complex and time consuming to write even the most basic script for it which I will demonstrate below with KM3's version of simple k replacer.

Aegisub

Here is Aegisub's simple-k-replacer.lua script. The script is over 70 lines even without the copyright / license agreement and the configuration window part of the script.

--[[
 Copyright (c) 2005, Niels Martin Hansen
 All rights reserved.

 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are met:

   * Redistributions of source code must retain the above copyright notice,
     this list of conditions and the following disclaimer.
   * Redistributions in binary form must reproduce the above copyright notice,
     this list of conditions and the following disclaimer in the documentation
     and/or other materials provided with the distribution.
   * Neither the name of the Aegisub Group nor the names of its contributors
     may be used to endorse or promote products derived from this software
     without specific prior written permission.

 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 POSSIBILITY OF SUCH DAMAGE.
 ]]

-- Aegisub Automation factory-brewed script
-- "Basic \k replacer"

-- Use karaskel.lua for skeleton code
include("karaskel.lua")

-- Define the name of the script
name = "Basic \\k replacer"
-- Define a description of the script
description = "Makes basic karaoke effects. Replace all \\k tags (and variations) with a custom string, where some variables can be substituted in."
-- Define the script variables that can be configured graphically
-- This is the primary point of the script, being able to configure it graphically
configuration  = {
	-- First a label to descript what special variables can be used
	[1] = {
		name = "label1";
		kind = "label";
		label = [[Variable-names are prefixed with $,
expressions are enclosed in % pairs.
Variables:
  $START = Start-time of syllable (ms)
  $END = End-time of syllable (ms)
  $MID = Time midways through the syllable (ms)
  $DUR = Duration of syllable (cs)
Calculation example:
  \t($start,%$start+$dur*2%,\fscx110)
  \t(%$start+$dur*2%,$end,\fscx90)]];
		hint = ""
		-- No "default", since a label doesn't have a value
	},
	-- Then a text field to input the string to replace \k's with
	-- Make the default a "NOP" string
	[2] = {
		name = "k_repstr";
		kind = "text";
		label = "\\k replacement";
		hint = "The string to replace \\k tags with. Should start and end with { } characters.";
		default = "{\\k$DUR}"
	},
	-- Allow the user to specify whether to strip tags or not
	[3] = {
		name = "striptags";
		kind = "bool";
		label = "Strip all tags";
		hint = "Strip all formatting tags apart from the processed karaoke tags?";
		default = false
	},
	[4] = {
		name = "workstyle";
		kind = "style";
		label = "Line style";
		hint = "Only apply the effect to lines with this style. Empty means apply to all lines.";
		default = ""
	}
}
-- Mandatory values
version, kind= 3, 'basic_ass'

function do_syllable(meta, styles, config, line, syl)
	-- text is the replacement text for the syllable
	-- ktext is the karaoke effect string
	local text, ktext
	
	-- Prepare the stripped or unstripped main text
	if config.striptags then
		text = syl.text_stripped
	else
		text = syl.text
	end
	
	if syl.n == 0 then
		return text
	end
	
	-- Add the variable names to the syllable data
	syl["dur"] = syl.duration
	syl["start"] = syl.start_time
	syl["end"] = syl.end_time
	syl["mid"] = syl.start_time + syl.duration*5
	
	ktext = config.k_repstr
	
	-- Function for replacing the variables
	local function var_replacer(varname)
		varname = string.lower(varname)
		if syl[varname] ~= nil then
			return syl[varname]
		else
			aegisub.output_debug(string.format("Unknown variable name: %s", varname))
			return "$" .. varname
		end
	end
	-- Replace the variables in the ktext
	ktext = string.gsub(ktext, "$(%a+)", var_replacer)
	
	-- Function for evaluating expressions
	local function expression_evaluator(expression)
		chunk, err = loadstring(string.format("return (%s)", expression))
		if (err) ~= nil then
			aegisub.output_debug(string.format("Error parsing expression:\n%s", expression, err))
			return "%" .. expression .. "%"
		else
			return chunk()
		end
	end
	-- Find and evaluate expressions
	ktext = string.gsub(ktext, "%%([^%%]*)%%", expression_evaluator)
	
	return ktext .. text
end

function do_line(meta, styles, config, line)
	if config.workstyle == "" or config.workstyle == line.style then
		return karaskel.do_line(meta, styles, config, line)
	else
		return { n=1, [1]=line }
	end
end

Wow that's a lot to take in for someone who hasn't coded before. To use this script, load your karaoke timings into Aegisub and then load the script into Aegisub's automation manager. Click "Apply Now, enter a template into the configuration window and then click "OK".

Karaoke Maker 3

Karaoke Maker 3's simplekreplacer.km3 scipt is a lot shorter at only 18 lines.

/* This is a simple k tag replacer script.
    -The example changes the border from 2 to 5 for each syllable.
    -To change the effect simply replace the '\bord2' and '\bord5' with the effect you want.
    -All strings should be surrounded by quotes.
    -All km3 tags are surrounded by square brackets. */

Event main {}  // Use the default event. (http://perseus.draders.com/km3/events.php)

Template simplekreplacer () {
    template = "{\bord2}{\t("  [0]  ","  [1]  ",\bord5)}"  [s];  
    // '[0]' is the start time of the syllable (ms).
    // '[1]' is the end time of the syllable (ms).
    // '[s]' is the actual syllable.
}

Run {
    main.simplekreplacer();  // Generate the simplekreplacer template using the main event.
}

If all comments are removed it's only 9 lines.

Event main {}

Template simplekreplacer () {
    template = "{\bord2}{\t("  [0]  ","  [1]  ",\bord5)}"  [s];  
}

Run {
    main.simplekreplacer();
}

To use this script all you have to do is import it into km3 via the "File" menu along with your karaoke timings and press the "Make Karaoke" button.

If you choose to save your loaded karaoke timings, the km3 script will be saved right into it so you will only have to load one file next time.

Conclusion

Karaoke Maker 3 was designed to have most of the functionality needed to make karaoke built in allowing you to concentrate on the effect, where as Aegisub's automation forces you to script this functionality yourself or find scripts made by someone who has. Although this allows for greater customization, it's not very friendly to those who have zero programming experience and can take a while to build up a library with similiar functionality as KM3.