How do I role play a level zero commoner in third or second edition D&D?

You don’t. Commoners are NPCs. I guess you could theoretically use the stat generators in the DMG to roll some up, but it’s not like you’re going to take them on a dungeon crawl.

Occasionally when a party is in terrible need of a hint, I may send in a wandering NPC that might say something that gives them a little nudge in the right direction. In open country NPC’s give information on traveling between parts of the quest if it is a protracted campaign. Since they have only one hit point (or 1d4 as I’ve also seen) they can be as numerous or few as you care to put them in.

A little looking turned up this which expands a bit on the 0-level Apprentice rules in 3E.

I seem to recall 1st Edition had some rules for commoners, but it’s been a while so I might be wrong.

@majorrich Looking at the stats of a common housecat, a ½ HD monster, I would go with 1d4 for commoners myself. And I’d like to think that that drunken blacksmith has more hit points than the glass mug he’s drinking out of.

Lulz. At level 0 a sneeze knocks them out. But there are consequences behind the screen.