Re: Where to find magical Diadem 3socket?
As for drops, you'd require monsters that drop from TC 87 so you'd want at least lvl 85 areas if not gambling.
Diadems don't spawn with automods so the best you could find would be a 15%ED 15%Max Dur 3os Superior.
If you are sure about magical, that's pretty rare as getting the proper mod for 3 sockets alone is rare, let alone a useful mod to pair with it.
Overall, gambling would be the best bet.