This reminds me of Jim's ccw class. I am so used to it by now, I don't even think twice each time. Any stranger approaches within 10 yard (considering we are in a wide open space where he/she/they could have take a different path), you point at them and ask politely, 'hi, how can I help you.' The point exercise works so well. It stops people and let them know that you are aware of them. In most cases, they just shrug off and change their direction to avoid me.

I imagine by doing this, I would have prevent the three guys from being too close before I find out what's their intention.