There are instructions in the users guide for downloading your custom built coasters from the earlier versions and then RCT3 will convert them for you and you can ride them. I never tried it, so I am not sure how it works, but the instructions are in there somewhere.
Yes, coasters work great. Just click "New Attraction", and then the Wrench icon. You can then search your hard drive for RCT1 & RCT2 track files. Just click and drop. It keeps all settings from RCT1&2, including colors and car layouts. Be sure to test the old creations, as the physics changed from 1&2 to 3. Be default a 'Low Friction' option is enabled on imported tracks.