Actually, Tuidjy has said that monsters never had triple hp, and since I was not there then I will bow to his knowledge.
In this case, he is incorrect. I am 98% positive that monsters did have triple HP.
Yes, I was mistaken. I just went to some of my old play-thoughts, and saw that ever since insane came around, monsters had triple hit points. There was a time when x2 was the biggest bonus (May), and that's what I must have been remembering. As I was reading the play-thoughts, I noticed that at least some AI units had quintuple hit points, so in this respect, less has changed than I thought.
But my point remains - enemy AI heroes have actually become stronger, and so have units, with their additional traits. Its the AI economy and infrastructure growth that have nose dived.
Here's a quote from my first play-through on insane (.915, June), when Frogboy thought the AIs were going down too easily for Insane. "Yithril & Magnar were definitely Insane, because they had the x5 hp and x4 population/research/production. [snip] As for the rest, with Resoln's capital at 509 population on turn ~50, I do not think they are at default either."
Nowadays, AI cities are capped by food much more easily - I think they lost some of the food bonuses.
The neutral monsters have mostly lost hit points - for example the cave bears have gone from 111 to 80. And Torax no longer has 960 hps.