The shuttle run is fairly simple. From the NFL website:
"The short shuttle is the first of the cone drills. It is known as the 5-10-5. What it tests is the athlete's lateral quickness and explosion in short areas. The athlete starts in the three-point stance, explodes out 5 yards to his right, touches the line, goes back 10 yards to his left, left hand touches the line, pivot, and he turns 5 more yards and finishes.".
The 3 cone drill is slightly more complex. Again, from the NFL website:
The 3 cone drill tests an athlete's ability to change directions at a high speed. Three cones in an L-shape. He starts from the starting line, goes 5 yards to the first cone and back. Then, he turns, runs around the second cone, runs a weave around the third cone, which is the high point of the L, changes directions, comes back around that second cone and finishes.
These drills, then, tend to measure slightly different things. Wide Receivers and cornerbacks tend to dominate the shuttle run, and strong performances in the shuttle run can help boost a player's draft stock. For example, Desmond Trufant's 3.85 time in the shuttle run helped make him a first round pick in 2014. However, not all players who perform well in the shuttle are selected in the first round; in 2011, Austin Pettis ran a 3.88. Tavon Austin, one of the more well known pure athletes in recent years, clocked in at 4.01 seconds in 2013, a performance which helped secure his selection in the first round.
The 3 cone drill, like the shuttle run, are typically dominated by Wide Receivers and Cornerbacks, who tend to be the fastest players in the draft. While an elite time in the 3 cone drill doesn't necessarily mean a top tier prospect (the top 2 times are Jeff Maehl at 6.42 seconds, and Buster Skrine at 6.44 seconds), This drill, like the shuttle run, can be useful tools to analyze a player's pure athletic ability.
These drills are useful tools to analyze athletic ability among all positional groups. While cornerbacks and WRs may post the fastest times in these drills, all players need the tools measured by them. They are useful drills for analyzing an offensive line prospect's mobility, for example. If an offensive lineman performs well in these drills and was previously rated as one of the more athletic offensive linemen on your board, then your rating is confirmed. If a prospect outperforms your rating in this drill, then it's time to watch more film. These drills are a bit harder to prep for than the 40, and measure a few more variables. While these drills won't make or break a player's draft status, a solid performance can positively impact a player's draft stock. However, these drills can't erase a player's onfield performance in college.
Let's take a quick look at guards in 2013. Larry Warford finished 8th among guard prospects in the 3 cone drill and 16th (last among participating guards) among guards in the shuttle. However, Warford has emerged as the best (some would call him 2nd best behind Jonathan Cooper) guard from this draft class. Clearly, combine drill results don't give a complete picture of a player's ability, and must be looked at in conjunction with film.