First of all, i have not played these games, my wife does, so if my question sounds strange, that's why. ;-)

We got the first two episodes on Desura, but i managed to snatch the whole four episode bundle during the insomia sale. So i now want to know, do these games rely on savegames from previous episodes like the Walking Dead games from Telltale, or can my wife just jump straight into episode 3?

Cognition doesn't use any information from previous episodes. Your wife is free to jump right into the third part without any savegame hassle!

Thanks for the info!