Gollum was once a normal hobbit like Frodo and Bilbo- yet after acquiring the Ring he became completely different. Why didn't this happen to Bilbo or Frodo? Yes, they were changed in some way, but it was not such a huge difference as in Gollum. His physical and mental state were drastically changed.
Gollum possessed the ring for nearly 500 years, from when he recovered it from the river in TA 2463 to when Bilbo stole it from him in TA 2941. During that time period he often wore and doted over the ring. In contrast, Frodo only carried the ring for less than a year, from April of TA 3018 to March of TA 3019, and specifically avoided putting it on during that time, having been warned of the danger. It just didn't have time to affect him to the same degree, though it should be noted it still managed to corrupt him to the point where he was unwilling to give it up.