Hearing running water sounds coming from hot water heater is there cut off safety switch inside the heater that's tells it is full ,its only 3yrs old 40 gallon tank

No shut off, it doesn't fill like a toilet. If you hear a "water sound" it could be from a leaking fixture or if an electric heater, you may be hearing the water boiling at the heating element. Turn off all fixtures in the house and go look at the water meter. Is the little red pointer arrow moving?

I changed the pressure release valve because it was bad it was letting water pass through.turned main water back on and red pointer is still moving and I can hear the water coming into the tank hours later

No shut off, it doesn't fill like a toilet. If you hear a "water sound" it could be from a leaking fixture or if an electric heater, you may be hearing the water boiling at the heating element. Turn off all fixtures in the house and go look at the water meter. Is the little red pointer arrow moving?

I changed the pressure release valve because it was bad it was letting water pass through.turned main water back on and red pointer is still moving and I can hear the water coming into the tank hours later

Then you have a leak somewhere.