Help with making AI

There are many ways to make AI, and to modify it to attack players when certain conditions are met, from collision with invisible hitboxes to more complicated stuff, Whats the easiest or least event consuming way to create Triggers based on distance?

And I want the event to be so that when the condition is met, one of 3 actions will be executed, not the 3 but 1 of the 3, how can I make this?

you can compare the distance every tick and execute the action when the condition is true ? there is no more optimised way , because i you use the sight of view or collision test , any kind of test in a game engine works in loop ( every tick ) or every milliseconds , so it's the only way to make a test