There are a lot more than three. I'd imagine for your circumstances
JBoss is the only real option, since it is free where the others are very expensive. Its also a little less rigerous when it comes to the common descriptor errors you tend to see in beginner's code - Weblogic and particularaly WebSphere are much more complex and difficult beasts.
For a more complete comparison of most of the app servers avaliable see
here.