There is no comparison between RCT and SM or NL. RCT is a GAME. SM & NL are roller coaster simulators. There are no goals, no people, no shops, no other rides. They are strictly meant for creating realistics coasters. RCT is a game with goals and objectives. Even playing in sandbox mode, you have the needs and wants of the peeps and employees to worry about. RCT is not trying to be a simulator. It's supposed to be a fun game where you can create an entire themed park. If you like having fun building themed parks, get RCT. If you enjoy creating realistic coaster alone, then get SM or NL. Otherwise, comparisons just can't be made. They're in two different leagues.
Oh, and yes RCT3 is good. In fact I think it's pretty amazing! It will be close to perfect once the first patch is released to fix a few minor issues. I've had it since last Thursday and am in awe with the different features I discover every night. It's awesome!