Dr.Mundo the doctor of hell! Is a very effective dps for 3v3, he can solo if you want, but he is best with an ally. First take Infected Cleaver and finish it first, it can fast be your best way to kill your enemy. Always use Masochism when in combat, it will increase your DMG and lead you to a easy kill.

As Dr.Mundo I use only AD runes, to maximize the DMG of Mundo's basic attack.

As Dr.Mundo I always go 27/0/3 to maximize your entire DMG.

First take Dorans Blade for FB because Mundo gains extra HP and AD. He do lots of DMG when first slow the enemy. Right after D-B buy Berserker's Greaves, so you can run after them.
Then buy Bloodthirster to get AD and some Lifesteal, Mundo can always use Lifesteal, because all his abilities cost HP. Get Infinity Edge to do maximum DMG to the enemy, and get some crit.
Right after Inf. Edge buy Black Cleaver, so you can get some attack speed, and some more attack DMG. Normally the game would be done now, but if it still going on, buy Phantom Dancer so you can run after them, get some more attack speed, and some crit to your Inf. Edge. Now you can choose between Bloodthirster again or Madred's, I prefer Bloodthirster first, because of the attack DMG. And Madred's after to finish off.

First take Infected Cleaver to slow the enemy and DMG it. Next take Masochism to get extra physical DMG. And then just go Infected Cleaver to rank 5 at lvl 9 and get Masochism to rank 5 at lvl 12, and then you can start to take Burning Agony at lvl 13.

I prefer take Exhaust to reduce the enemies DMG on you, and slow it too.
And a perfect combo to Exhaust is Ignite, which DMG overtime and reduce the enemies health regen.

At all Mundo is a good DPS and tank, you can play him on many different ways, you can go attack speed, AP, AD, tank and much more. But the most effective way with Mundo I found, is the AD way to the glory.
You can easy win with a DPS Mundo on your team, if you got some okay teammates who can plays their character.

DPS Mundo can works in the jungle too, then you will need to go up in Burning Agony ealier, and get the Bloodthirster fast as possible to regen. the HP lost in the jungle.

Mundo is not very effective in team matches, because his Cleaver cannot hit the enemy if they are grouping up. But Burning Agony in the higher lvls is really effective, plus his Masochism to do the extra DMG on a target.