No antennas does work with 3g, but it only shows you the one tower you are currently connected to at that moment in time. On gsm it shows all towers near you that the phone sees. It's still just as useful on 3g though, I use it all the time.
An even better app is called rf signal tracker. It actually records you on a map showing all cell sites you've connected to, and you can play it back like in real time. It's great. But it uses lots of resources, so don't force close it. Just hit wait and let it do its thing and process the data.