I honestly don't remember them putting out a lot of dev material, if any at all. It was pretty much the announcement, some screenshots, the trailer, hands on at E3 or with some game sites, a demo and then it was out. I remember staring at these screenshots for an unhealthy amount of time,
http://www.gamespot.com/articles/rollercoaster-tycoon-3-preview/1100-6092089/
I know this isn't early footage but it's fun to see. Here was the first trailer shown for the game, (horrible quality)
[video=youtube;vC_nc7LwGWw]https://www.youtube.com/watch?v=vC_nc7LwGWw[/video]
Lots of memories back then on the forums where everyone started fighting about how cartoony it was with comparisons to Theme Park World/Sim Theme Park. RCT3 definitely began the split down the base on how an RCT game should look. It came out when I was starting college and my roommate laughed at me for playing a kids game. Ha! [heart]
Also when the demo came out it didn't run very well which caused the community to roar some more. I think Frontier put out a performance patch for the demo? Might be wrong.
https://www.youtube.com/watch?v=R-HuhgQbEHU
I think Frontier did an amazing job taking their cartoony look in RCT3 and blending it with realism for Planet coaster.