Steve Hassenplug contributed this modified driver written by Dick Swan, to continuously ping several Ultrasonic sensor at a time, and store the reading in the SensorValue[] for the program to use in the normal way.
The user program should include:
#include “NXT 3x Ultrasonic Driver.c”
to load the driver, and
StartTask(UltraSonic3xDriver);
to start the driver.

The number of US sensors, and the ports can be configured in the first few lines of the driver.

Download driver here.