Raw was three hours last week because it was the 15th Anniversary Show. It's normally two hours.

In the mid-late 90's when the then-WWF moved from one to two hours (9-11pm), for some reason they gave the first hour the title "Raw is War" and the second hour "War Zone." Why they did that, I'm not sure the "official" reason.

I would guess it was probably done to enhance the ratings against the WCW Monday show, Monday Nitro, which ran from 8-10pm. Separating the hours would allow the second Raw hour which was not against WCW (10-11pm) to stand on its own in the ratings instead of being averaged with the potentially lower first hour of Raw which would be competing against the main event on WCW.) WCW led the ratings overall for 80+ weeks starting in 1996, so it's safe to assume that the split of the WWF show into two separate programs was to look as good as possible compared to the then top-rated WCW Monday Nitro. Why Nielson wouldn't say "That's cheating," I have no idea. Due to how the ratings are split up, the WWE does usually have a "big" segment at the end of the 9-10 hour to draw in viewers to bump the last quarter of the 9-10 rating, and have a strong first quarter for 10pm hour.

I watched at the time, so I really don't remember, but I've read that the WWF changed the ring apron for the different hours to include the name of the program ("Raw is War" to "War Zone") which is something they don't do anymore, though the hours are still split. Wikipedia says that graphics on episodes since 2001 show the first hour as "Raw" and the second is "Raw Zone."

Last week's RAW was a 3-hour show to celebrate 15 years on the air.

Well, last time I watched Raw, the two hours had different TV-Whatever ratings. That might be why they're split in the Nielsens, too.