*The biggest change from the prequels are the new three-dimensional graphics, as per the change of the main development team. While this does look nice and makes the game look good, they can really slow the game down, especially if there are many objects on screen. Besides, a game's quality should never be judged by graphics alone. Gameplay and control are the main key factors to look to. The previous two run so fluidly (despite 2 having slow loading times), and rarely would there be lags.
*Because of this, you can now zoom in much more, making it easier to zoom into certain areas or angle the camera so you can edit them.
*You can set up the "Coaster Cam" which allows you to view many rides from the front vehicle, and gives you the interpretation that you're riding the ride itself.
*The menus are re-arranged, so you will have to get used to the new HUD if you're familiar with the first two games. With this new menu system, however, everything is grouped better, but it can be a little slower trying to find everything.
*Landscape editing tools are now much more advanced, allowing you to form perfect hills and chasms. Of course tile-based landscape editing is back, but now that the ground can be any level and not multiples of five as it was in RCT2, you have to be much more precise if you make a mistake with the landscape, or you risk screwing up paths that you construct on the surface (as they may not connect properly).
*This can make underground building a little irritating at times, especially considering that the ground will auto-adjust if you attempt to build a piece of track underground through the surface but not with paths, especially if you're unfamiliar with the new non-surface path editor.
*The guests have received a significant upgrade over the previous games. Not only are they given new appearances, with them able to have genders, age ranges, hair and clothing styles and colors, and various skin colors, their intelligence and actions are much, much higher and advanced, as they have higher tolerance for rides and many other traits (etc. they don't vomit often, they don't turn into dead ends or head into a shop w/o purchasing anything, they don't form straight lines if multiple are on a single pathway, they rarely complain about being on rides for too long, and they are very unlikely to become vandalizes). Should one end up in water, they will swim to shore rather than drown, and are also immune to death by being caught in an accident. They also will ride transport rides for the purpose of transporting, not just because they think it's a good fun value.
*The third game also spices up the intelligence of the staff (Handymen/Janitors, Mechanics, etc.), so they will respond better to problems in the park.
*When a ride breaks down, you are now told instantly on the message the cause of breakdown rather than having to go to the ride's statics menu. Each one can also be customized individually.
*Ride queues are much more customizable, allowing you to change the colors of the primary and secondary colors of the standard tiled queues, as well as their rails.
*Track designs can no longer be saved with scenery, making it much like the first game rather than the second (however I can't conform as I only ever used RCT1/2 track designs),
*Track designs from RCT1 and 2 can be imported into RCT3, a feature that RCT2 lacks with the original.
*Buildings seem to be more difficult to create (I'm sure they exist; I just never tried them for myself.)
*When manipulating land, the cash spent is all done at once, therefore you can check the prices before making the edit.
*Guests can now buy multiple things from one shop/stall, including toppings for that stall's particular food.
*Mechanics can repair vandalized objects without the player having to replace them by hand.
I honestly wouldn't mind a 2D version of RCT3 so I can still get the same experience that RCT3 offered with the great visuals of the original two games, plus with hoe fast they could run even on older operating systems. The leap to 3D is what killed RCT3 for me since it made the game run so slow.