I know this is not the right place to answer, but here we go...
I had myself nat 3 and DNS issues months ago. I tried to do everything manual, following several tutorials, but the magic wasnt happening. The most curious and annoying is that my modem was wired to the PS3, no wireless, no router wi-fi, no nothing.
The problem ended when i quit of try to solve the problem and bough a new modem, witch is wired to my pc and at same time sends signal to any other wireless device, including my PS3. Also i had to configure everything manualy, to be sure to have the finnest connection.
In your case, if you dont want to buy a new modem, i suggest you type in the google "PS3 + NAT 2 or 1 (depending if its wifi or wired) + your modem model". Also do it on youtube, probably you will find video tutorials there also.
Im just answering because i respect people who take care of have a good connection to play. Some people dont have this kind of care.