I will try to answer a question here on the forum for the first time, hope I don't embarrass my self now. :smiley:
Yes, 3 puls will increase the base defence of the item with 90%, which of Black Hades is a Chaos armor. The Black Hades can have 140-200% enhanced defence, so your 90% is added to that percentage (and not to the total defence of the Black Hades itself, which is easy to believe).
As the Black Hades is a unique exceptional armor, it can also be upgraded to an elite, giving it more defence (and also higher requirements).
Other socketing than pul to increase defence I let be unsaid, but I am sure there are some good jewels you can find that both increases the defence and adds some other good stats.
If you're on Ladder, cube 2 Puls into an Um. Take the Um, and along with the other remaining Pul, and make a Duress out of an elite armor. Now you have a high defense armor with some nice offensive mods. If you want even more defense, use the Um and Pul to make a Stone.
Depending on your funds, also consider eth skullders, (eth or noneth) steel carapace, gladiators bane and templars might. (maybe even upped black hades?) They might end up having higher def than the black hades. And also consider if ed% is the only thing that is important for your character
edit: sorry for not mentioning, but jewels can get to +100 def, don't know if it also spawns on rares though, considering black hades base def of 343 the added def from pul would be: .30 * 343 = 102.9 so pul is a little better unless the jewel has other nice mods
edit2: ok just checked my story and arreat summit now lists max of +64 def on jewels, I guess the ones that I have are from .09 or something
Just my opinion but, I'm pretty sure you can put those 3 puls to much better use. Even on NL. Just buy a better armor with higher defense and better mods (like duress). Someone will likely take the offer.