This is a build for Trundle mostly in 3v3, but can also be played in 5v5 (then you may need to get some more life/armor)

The crit runes is to get the high crit number in late game, and to make some crits before phantom dancer. The health is to survive longer in the team fights at start, and the cd reduction runes, is to constantly make contaminate, and to make the double strike with rabid bite.

Ghost is for capturing up on somebody or for fast escape, and exhaust is for ad champs like trynda. If you want to go jungle in 5v5, then smite is a good option.

The masteries is to do as much damage as possible and to get in higher lvl as fast as possible

Just go farm whenever the chance is there and it dont **** up the team.(ofc if you are losing go help your team)

you start of with vampiric scepter to stay in lane for as long as possible, at least long enough to get wriggles lantern. Now you can start farming minions including dragon, lizard and golem pretty fast, do it. Then get mercury treads, so when you uses contaminate the crowd control is reduced ekstremely. Now you grap the phantom dancer to make the crits more often, but you still need to farm minions, and maybe do some killing champions if you are higher lvl (which u should be) then you go for the infinity edge to do some killing blows in team fights and 1on1. The last items depends. If the other team got a armor tank go for Last Whisper. If you wanna be kinda tanky, go for Warmog and/or Frozen mallet to get enough life to tank. And again if the others have a guy like trynda or another ad champ, maybe grap thornmail/Sunfire cape. If the others have ranged AP champs, banshees veil is a good optioon. If its a long match sell wriggles lantern and get some lifesteal in bloodthirster.

Pros:
you can make HUGE damage vs everybody because of your anti-tank ulti.
You can take F.eks a malzahar stun which normally is 3 sec and reduce it to a stun under a sec.
You can escape nearly every gank, and You can escape every single 1on1 fight (if u start with full life or something close to it)
Cons:
You can get Nuked in 2 sec early game.
You are vulnerable late game and cant do much if focused in 5v5

Finally i wanna say that if you go this build, it would be good if the game is over after half a hour in 3v3.