How many of these people have actually played all 3?
Hellgate was released unpolished. Full of flaws. The maps are recycled making things very reptitious. There are hardly any skills. Most of them are passive so there is not too much flexibility. For the FPS game style, it's quite addictive at first. Because the lack of skills/maps that excitement dies out mighty quickly. I don't recommend investing money into the game.
Mythos, I think people are getting the wrong impression. There are better graphics out there. The graphics really aren't much worse than WoW, or Fate, or Titan Quest. Pretty much on par. The graphics ARE better than Diablo2. The game is going through so many changes and evolving so much. That if you had a chance to play the beta a month ago. It's going to be completely different on the next patch. They're implementing an overworld, crafting, more interesting quests. Etc etc. I personally like Mythos, and I'm excited for the next patch.
Mythos has alot of similarities to Diablo. Same interface. Same skill tree. AOE is a very important aspect. Blue Items, Purple Items, Green Items, etc etc. Stat points, skill points. Similar weaponry. You'll instantly notice a lighter mood. Sort of a cartoony effect. Not a serious, more of a get out there and farm.
Mythos is a student that will never surpass the master. Its pretty much a clone. I do think its fun though. I just like diablo 2 better. No midgets or ram people.