jtk, your last post is so rambling that I don't even know where to begin answering it.
Let's go back to the beginning. We are at some altitude, say x, and we need to be at altitude y at some point in time/space. In order to reach that point, there's a few things we need to know. Top of the list will be the TOD time/place, which is the time/place we need to start our descent. Secondary to that are the ROD and speed numbers but what you will find is that the ROD and speed in the descent are a given for your aircraft type and passenger comfort. They don't vary (much) between flights.
For your average spamcan, this will mean 100 knots and 500 feet ROD, more or less. For your average pressurized airliner, this will mean 250 knots (below 10.000 feet due to speed restrictions) and 1500 fpm. Both will lead to a glideslope of approximately three degrees. And if you have anything in between (like the C414A of a friend of mine, who I spoke to just last weekend about this) you do the calculations once and then you know the ROD to achieve for your typical cruise descent on a three degree slope.
That leaves the TOD point as the only variable to calculate. That's where the 3X rule comes into play. And that's what A&C argues: the rule can be applied to anything that flies, as long as the objective is to have a three degree glide slope.
Again, how to achieve and maintain that three degree glide slope, varies from aircraft to aircraft. The faster the cruise speed in the descent, the higher your ROD. But three degrees is doable in almost any kind of aircraft (gliders, space shuttles and the like excepted).
Oh, and my calculations are simply based on working knots, miles, feet, hours and minutes back to metric values. Losing 5000 feet takes 15 nm according to the 3X rule. 5000 feet is 0.82 nm. arctan (0.82/15) = 3.13 degrees. Close enough. So the 3X rule leads you to an almost-perfect 3 degree slope. (Your example of 10.000 feet in 30nm works out the same by the way.)
ROD at 95 knots: 95 knots divided by 60 is 1.58 nm/min. The glide ratio was 5000 feet per 15 miles, so the ROD is 1.58 * 5000 / 15 = 527 fpm. That's just a number you calculate once and then use every time for that aircraft type. In fact, you will probably work out the exact RPM (or MAP) reduction you need for that ROD and remember that - see italianjons post.
ROD at 300 knots: 300 knots divided by 60 is 5 nm/min. Same glide ratio, so the ROD is 5*5000/15 = 1667 fpm.
Now we both got those same answers. So why do you say:
All I am trying to get across is that it clearly is a good rule of thumb for faster aircraft but is mathematically incorrect for smaller one.
What is wrong with a 527 fpm ROD at 95 knots? Or indeed even a 333 fpm ROD at 60 knots?