I have a ping sensor hooked up to my ardunio with a servo And all of this all works accordingly when the USB cord is plugged in.
when I plug in a external power supply (ac adapter) to ardunio only servo works with the code the ping sensor does not blink and no I don't think it's a power issue the adapter is 5 volts at 3 amps I heard somewhere about a pull-down resistor or something, about there being a flaw with the board.
You have the 5VDC going into the barrel jack? You won't get 5 V out that way, the regulatar will drop 1.5-2V on you, so everything will be underpowered.
The 5V will need to to into the 5V header pin.
Yes, you can connect your 5V source, or three 1.5V batteries, to the 5V pin on the power header.
The 16 MHz arduino will run well on 4.5V.
6V is too much.