You don't know what you knew as a child. It's entirely possible you knew nothing, as you stated.
But atheism is a belief. A religion. It posits that there is no God. Thus, it's more accurate to say that you were born an agnostic. (After all, you were born not knowing about Los Angeles, but that doesn't mean it didn't exist and that you believed Los Angeles didn't exist. Chances are, if your father told you at 3 that you and the rest of the family were going to Disneyland in L.A. that you would have been ignorant of L.A., but nothing more or less.
You can say that you were ignorant of Santa Claus and the Easter Bunny at 3, but that didn't mean they did or didn't exist. It simply means that you didn't know enough to have an opinion. That came later.
Some people who have near death experiences say they saw people they knew before they were born, and that they had a clear memory of knowing them; however, at birth that knowledge was not part of their mental make up.
The bottom line is that you were born an agnostic and then taught things that were both right and wrong. If you argue that God is one of them, so be it. I believe the case for God is considerable. The Earth's exact placement in the solar system, the existence and placement of the Moon (a foreign body to our solar system); the fact that the Moon is the exact size, mass and distance from the Earth to keep it stabilzed, and the exacting complexity of advanced life forms all attest that there is a God. If the Earth was any closer or further from the Sun, life would not be possible. If the Moon didn't stabilize our orbit, we'd have a new equator every day. Amino acids can sit around for untold millenia and not form anything other than a puddle, so who kicked things into being?
I don't begrudge anyone being an atheist. It's a free country (for now). But atheists shouldn't treat God like Santa Claus and the Easter Bunny. The very fact that many billions of years ago, the Universe was not...then suddenly it was. It's not static. Babies are taught what they're taught. I'm not saying the Hebrew God brought everything into existence (though I personally believe it for other reasons), but something did...some Intelligence, and those of us who believe in God are not the idiots Bill Maher and Christopher Hitchens make us out to be. So show me a baby who's convinced there is no God, and I'll show you a baby who doesn't believe in Los Angeles!