Multiple HC-SR04 with one 5v 7805 regulator
You really could benefit from a DC/DC switching power supply.  For instance you are wasting about 20% of the power of the 7.2V battery and something like 50% of the power of the 11.1V battery.  

That said, could it be that you are triggering the 3 ultrasonic HC-SR04 too closely together?  If you don't wait for the sound to die out from the 1st sensor the 2nd sensor could give unexpected results.  Same goes for the 3rd sensor.