Multiple HC-SR04 with one 5v 7805 regulator I am working on a autonomous track vehicle. I have mounted 

3 HC-SR04 sensors
1 Sharp infrared distance sensor
3 infrared led distance sensors

1 Arduino Mega 2560
1 H-bridge Board HG7881

I am trying to use 7.5 v 1700 mAh Nicd battery (also tried 11.1 Lipo 2200 mAh)
For all sensors power (5v) I developed 5v regulator using 7805 which is connected directly from battery as input. The output 5v is going to power distribution board (used from quadcopter extra board) which provides 5v to every sensor. 

The real problem starts here when I am not getting correct or nothing in reading from Ultrasonic sensors and my vehicle starts behaving abnormal. I checked that sensor reading is either 0 or wrong. I used both simple and with NewPing library.

I checked when using 7.2v battery the output from 5v regulator is 3.72v. 

Now my question is if the power is below 5v, does sensors starts behaving like this? How can we supply voltage so every sensor will receive full 5v to work properly?

My vehicle will not work until sensors provide correct distance reading. I worked successfully with one sensor but with number of these, tired working since many days.
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.