How come when I try to play Airport Tycoon 3 you cannot run multiple instances of this application at the same time message keeps on appearing?

The message indicating that you cannot run multiple instances of Airport Tycoon 3 suggests that the game has a built-in restriction to prevent simultaneous instances, likely to avoid conflicts with the game's data and resources. This is a common feature in many games to ensure stability and prevent issues like data corruption. If you need to run multiple instances, consider using virtual machines or separate user accounts, but be mindful of the game's terms of service.