Just a quick update.
The ISR code posted did not work as intended. It is still being modified.
However, I have now modified radar3 to show echoes with a shadow.
The length of the shadow is the number of 40 Khz pulses in the received echo.
The image shows the result of moving a can, Ø 5cm, across the three hc-sr04.
In short, the green color is the result of triangulating the right and center listener, the red is the result using left and center listener, and the blue is result from using right and left listener.
Also shown are some random echoes.