Just as an example...using this ranking (which by no means is this how I'd rank players, but at least it's a list to reference)...
http://www.cbssports.com/nba/playerrankings
5. Stephen Curry, 3 years in college
7. Paul George, 3 years in college
14. Damian Lillard, 4 years in college
22. Chandler Parsons, 4 years in college
24. Joakim Noah, 3 years in college
26. Isaiah Thomas, 3 years in college
34. Kemba Walker, 3 years in college
43. Wesley Matthews, 4 years in college
44. Tim Duncan, 4 years in college
45. David Lee, 3 years in college
50. David West, 4 years in college
So, if you just use this list as an example, only 11 of 50 guys went to college for 3 or more years, 4 of the 11 in the bottom 10 of the top 50. This means if you find yourself playing your Junior year in the NCAA, the statistical odds are extremely high that you're not going to make it.
Lets also include that this list DOES NOT include Kobe Bryant, Derrick Rose, Russell Westbrook and possibly others who do not quality to be ranked due to games played, but would clearly be in the top 50, pushing a few of these guys out.