Why are there three advertising channels (rather than two or four, etc)?
The three advertising channels are spread out across the BLE spectrum to minimize interference potential from other services. A BLE device advertises on an advertising channel for a fixed time and then invokes a random delay before switching to the next advertising channel.
Since battery life is a prime concern for BLE devices, adding more advertising channels could be problematic as advertising would then reduce battery life. More advertising channels would also mean fewer available data channels.
Using less advertising channels would increase battery life but it would also result in more advertising collisions between nearby devices. This would increase the latency of device connections.