Dugtrio is very hard to use... because he's so frail. You have to be uber awesome at prediction to be able to get him into play. His Defense is so bad, in fact, that after a Swagger, mine hurt himself... and actually OHKOed himself.
As for a moveset, I don't advise Jolly. Dugtrio needs all the Attack he can get, and can't 2KO Blissey when he's Jolly (not to mention it's usually a gamble as to outrunning other base 120's, such as Alakazam). Adamant is the way to go.
As for the EVs, max out Attack, 42 EVs in Special Defense, and the rest in Speed. With a little bit of attention to Special Defense, he's able to take on Ice Beam Blissey (granted he catches her on the switch, or switches into a Thunder Wave). And, amazingly enough, Dugtrio can survive an HP [Ice] from the likes of Jolteon.
So in total, here's the best Dugtrio:
Dugtrio @ Choice Band
Adamant Nature
EVs: 252 Attack, 42 SpDef, 216 Speed
- Earthquake
- Stone Edge
- Aerial Ace
- Sucker Punch
You'll rarely use anything but Earthquake with Dugtrio. Stone Edge is only helpful with a critical hit (mine KOed a near full health Milotic with a critical hit Stone Edge), and without it, Skarmory walls him to no extent. Aerial Ace is just to knock out Breloom (and, to a lesser extent, non-Choice Scarf Heracross), and Sucker Punch is amazing useful, being able to OHKO Gengar and Alakazam, and will KO a Starmie if it has around 80% HP left. Plus, Sucker Punch gives you the element of surprise when you're up against a weaker opponent who's faster then you; such as an Aerodactyl, Jolteon, or an Agility Metagross.
All-in-all, Dugtrio makes the best Blissey killer, and the best SkarmBliss killer killer. But again, becuase he's so frail, you've got to be real careful with how you use him. And I HIGHLY suggest running a Rapid Spinner with this guy, since continued Spikes/Stealth Rocks will really put a dent in him.