First of all i want to explain a little bit about what downhill actually is. It is one of the disciplines in alpine ski racing. Women and man, which are ski racers can ski it during the Alpine Ski World Cup. It is one of the most dangerous sports. I am going to explain later why.
While skiing downhill, a ski racer reaches speeds like 130-140km/h. That's why it is dangerous. But on to the topic. Let's consider a downhiller which is about to crash into the net. If there is just only one net, the speed is that high, that the energy breaks up the net and the ski racer is going out of the track, which might cause death. But now a situation, where we have 2 nets. It will stop the donwhiller, but it may cause a really bad injury. That's why we have 3 nets. It stops the ski racer and keeps him safe. Sometimes we have not 3, but 4 nets because of the audience, which stand next to the track.
Here are links, where you can learn more about ski racing: