"The http://elderscrolls.wikia.com/wiki/Frost_Damage_(Skyrim) enchantment is 25% stronger when placed on items crafted from Stalhrim. This effect is identical to the http://elderscrolls.wikia.com/wiki/Frost_Enchanter perk from the http://elderscrolls.wikia.com/wiki/Enchanting_(Skyrim) skill tree, or the http://elderscrolls.wikia.com/wiki/Augmented_Frost perk from the http://elderscrolls.wikia.com/wiki/Destruction_(Skyrim) skill tree."

That is what Elder Scrolls wiki states. My question is how exactly does this work?

Exactly what it says. If you would have been able to enchant 20 points of frost damage, you'd now get 25.

It's not clear if the three sources stack, and if they do, whether they each add 25% of the base value, or compound together. I.e you could get 75% more, or 1.25 x 1.25 x 1.25 which gives you over 95% more. Augmented Frost can also be at 50%, so potentially you could more than double the effect.

Not sure how it works here, but in most games adds are all based off the base damage, it would be weird if that wouldn't be the case here.