Historically, "stout" was "stout porter" (stout meant "strong" - so really just a stronger version of porter), and "mild" simply meant "not aged". Porter *didn't* arise out of the blending of three beers...
The "usual" differences often quoted are that dry stouts tend to be more bitter than porters and use roast barley, where as porter is less bitter (sometimes slightly more residual sugars) and doesn't use roast barley (often uses brown malt)... milds tend to be much less hoppy than either, and often (though not always) of lower gravity and slightly lighter in colour....... (NB: Patent malt is most definitely used in porters - in the 1800s when it was invented, brewers started using pale and black patent to replace old style blown (brown) malt as it was much more efficient and still gave roast flavours and the right colour.)
Of course, none of these definitions are strictly true - there is significant cross over between mild, brown ales, porters and stouts. The true definition of what style a beer is is entirely down to what the brewer calls it. (Hook Norton Double Stout is a good example - 5% Brown, 5% Black malt, so most would think from the grist that it was a porter - doesn't say porter on the label though
...)
If this is the same organisation I suspect you're talking about, they also land up classifying a saison as a best bitter... and then people wonder why it's cloudy and tastes funny...!!
it was just put to me with such gusto i was wondering what id done wrong,