Can't find last 1 of 3 npc to complete "Thane of Falkreath".

Speak to Runil to get a quest to retrieve a journal.
Speak to Dengeir of Stuhn to get a quest to kill a vampire
Speak to Hert at Half-Moon Mill to complete a favor (chop wood)
Speak to Mathies for another favor (pick crops)
Speak to Thadgeir to get a delivery favor
Speak to Dengeir of Stuhn to get a quest to steal a letter

Seems like someone copypasta'd a Wiki entry on your behalf

Bookmark these:

http://www.uesp.net/wiki/Skyrim:Skyrim

http://elderscrolls.wikia.com/wiki/Skyrim

And pay closer attention to the former.