Except a true 3 can't really guard a true 4. KD could guard a lot of different types of true posts. A true 3 really can't do much damage inside. KD could score it inside.
Again in college there are very few true anythings. That's why you won't hear a coach say a 1-2-3-4-5. He'll say a guard, wing, or post/big. Just because a guy is 6'8" doesn't mean he is a big just the same way a 6'4" guy isn't necessarily a guard. In the college game a lot of the two are swapped, there are a lot more 6'4/5/6" "posts" than there are 6'8" wings.
The following is from Wikipedia.
"Small forward
The small forward, also known as the "3", is typically somewhat shorter, quicker and leaner than power forwards and centers. Thus, in the NBA, small forwards range from 6 feet 5 inches (1.96 m) to 6 feet 11 inches (2.11 m). The small forward position is considered to be perhaps the most versatile of the main five basketball positions, due to the nature of its role, which is sometimes similar to that of a power forward, and other times more resembles the role of a shooting guard. The small forward and shooting guard positions are often interchangeable.
Small forwards have a variety of weapons, such as quickness and strength inside. One common thread between all kinds of small forwards is an ability to "get to the line" and draw fouls by aggressively attempting
post-up plays, lay-ups, or slam dunks. As such, accurate foul shooting is a common skill for small forwards, many of whom record a large portion of their points from the foul line. Small forwards should be able to do a little bit of everything on the court, typically playing roles such as swingmen but also as point forwards and defensive specialists.[3] Examples include LeBron James, Carmelo Anthony, Paul Pierce, Hedo Türkoğlu, and
Kevin Durant. Larry Bird and Scottie Pippen are examples of earlier players in the league's history for this position.