UW HM question

In order to unlock UW in HM do i have to complete all 3 main campaigns or just one? Thanks for your time.

Only on the town you leave off from.
So if you leave from ToA, you have to finish Proph

I'd imagine just the campaign you are entering it from.

In order to unlock Hard Mode in any campaign, you have to have beaten that campaign at least once on your account and have the character you're entering the Underworld on be level 20. So if you have beaten Factions, you can select the Hard Mode option in Cantha with any level 20 character, and if you access the Underworld from Zin Ku Corridor (the UW and FoW entry point from Cantha/Factions campaign) you can go in Hard Mode. If you have beaten Factions and try to enter UW from Temple of the Ages in Prophecies or the Chantry of Secrets in Nightfall, you cannot select Hard Mode.

Slightly off-topic, Underworld is an advanced area with a pretty steep learning curve for new characters. If you've never been there, I assure you, Normal Mode will be tough enough. Give yourself the chance to get familiar with the area before you arbitrarily decide to go in Hard Mode .