so i was thinking of buying RCT3: Soaked! but i was wondering do i need RCT3 to use RCT3 Soaked! ?

yeah, you could buy RCT3:Gold for 20 dollars (if your here in America and not Europe) for both rct3 and soaked. and if you want wild too, you could throw in an extra 10 dollars for RCT3:Platinum.