So, I'm trying to buy a house in Falkreath and hence become Thane.

According to the wiki, in order to assist the people of Falkreath, I can:

• Complete Miscellaneous Quests from the region.
  
• Complete Bounty Quests from the region.
  
• Complete A Daedra's Best Friend
  
• Complete Ill Met By Moonlight

So far I have:

• Completed a bounty (killed the dragon)
  
• Finished Daedra's Best Friend
  
• Completed Ill Met By Moonlight

And my assist count? Is 0/3 still. Is the wiki completely wrong or is this just another fine quest breaking bug brought to you by the fine folks at Bethesda.

The wiki is wrong, none of those quest actually help anyone in Falkreach.

I have complete 3 other misc quest here : bringing ashes to the priest (maybe the easiest quest in the game), finding a notebook for the priest in a random cave and stole a letter for the ancient jarl.

Fairly easy to be a thane here.

I don't believe there is a house to purchase in Falkreath, if that is the main reason you want to become Thane.
http://www.uesp.net/wiki/Skyrim:Houses

yeah Falkreath house costs 10 gold coins per day.

That is a "rental" and you don't need to do anything to use it.

Sarcasm is lost this days :p

sorry...missed it completely!...my bad

Ah thank you. I'll look for those quests to check out the sword at least.

Just an add-in because I just finished this. The miscellaneous quests will help you fulfill the requirements. They include cutting wood for the mill north of Falkreath (Half Moon Mill, I think is the name) along with picking lettuce/potatoes/gourds for Mathias.

I did a couple of bandit clean-ups for the bounty thinking that would help out, but it never registered. Helping Valdr with the spriggins is supposed to, but it didn't work for me (though maybe you have to meet him back in Falkreath for that to register. I didn't feel right breaking into Lod's house for the former Jarl, so I was looking for a couple of different options. I do think you have to return and talk to Lod after completing A Daedra's Best Friend in order for it to kick in.

Hope that helps.

Did return to Lod, thinking the same - no dice.