My D often ran away and had to be chased down at that age. She used to head down the block, with the knowledge that she was not to go around the corner (there was an alley halfway around the block, and cars came flying out of it without stopping). More than once she would head down the block with assurances that she would not go beyond the corner, then head around it full tilt. It was a full out sprint for me to catch her before she got to the alley. When she did that, she was taken straight inside with no more outside time that day. Later she was diagnosed with a non-verbal learning disability... I have heard that an inability to safely walk in parking lots & cross the street can be a symptom of that. No telling if that is why she had no concern for her own safety at three, though. Maybe it was just her being three.
Also, I recall that feeling of being caged when growing up. I remember a camping trip we took in the mountains when I was six, and how glorious it felt to hike and play in that much open space. I ditched my mom and hiked around a small lake (jumped a stream she did not want to cross, and just kept going). 45 years later it is still one of the best memories I have in my whole life -- cavorting all the way around this lake while my mom stood on the opposite shore shouting at me.
Maybe take your son hiking and camping if you can.
