Slightly. It will use some battery searching for 3G networks. But not much.
The real power drain is when USING 3G, ie. browsing or downloading. Not while simply being idle on the network.
If the symbol shows 3G, you are connected to an available 3G network. It will show E when 3G is not available but EDGE is. It will show Airport strength when you are connected to an airport network. It basically shows the fastest available network you are on.