First off we get to see a couple in love via internet. He's American, she is Egyptian and they have cyberlove for over 10 months. She gets infected and kills herself off. He is in shock.
Fast forward seven year and we get to hook up again with Justine (the youngling from part 2), who has now turned into a rebellious teen in a Amish society. Taken over by foster parents she isn't satisfied with the life she has. By chance she find an old decrepit laptop stowed away in a car. She unwittingly takes a risk and turns on the little death machine. After which she is instantly attacked by... a chat-room. There she meets Adam, who apparently lives in town (Houston to be exact) and invites her over to visit. Naturally she takes a trip by foot to meet the mysterious internet lover...
Not all is bad with this movie. Yes, some of the acting is off, but in general the action is good and after a while you get used to the crappy blue screen. If you take it at face value you might actually enjoy yourself, as there are a couple of really worthwhile sequences with the opening segment a standout. Additionally there are a couple of interesting ideas, like causing pain to spectres via ultrared light or the whole house in the prairie motive.
But if you put any thought into the movie it just doesn't make any sense. In the end it turns out that Justine's whole trip was pointless and gives nothing to the story. Additionally we are to believe that it took the army seven years to coordinate a global nuclear EMP attack (what does that say about the army?). Not to mention cell-phones that have power after seven years of laying around in the dirt (Nokia eat your heart out!). All in all I strongly doubt that after the prolonged period of lack of power sources (with no-one to run the power plants electricity would be down in a matter of days, save for maybe nuclear facilities), maintenance and so on anything electromagnetic would be working after seven years. Basically the whole premise stinks of crappy writing and zero afterthought as it really doesn't make much sense.
Generally if you were involved by the previous two parts you can watch this to get some sort of closure. But don't expect much (in any department, especially special effects) and you may just have a good time.