Thanks for checking out GXT 3! You don't need to do anything to move reset.css into your web directory - the compiler will automatically make sure it is there, provided you are using a theme that needs it (all themes extending from Base will have it).
In your module file (ends in .gwt.xml) you might have a rename-to attribute - if so, that is the name of the module. This is the folder in which the modulename.cache.js file is created, along with other things like a hosted.html file, and eventually several <giant-hash-name>.cache.html files. If you don't specify a rename-to, it will default to the path to your module. So a module file at com/my/project/MyProject.gwt.xml will be com.my.project.MyProject, and the css file will be loaded like this:
<link rel="stylesheet" type="text/css" href="com.my.project.MyProject/reset.css" />
As far as copying the jar into WEB-INF/lib/, this is only necessary if you use any GXT classes from your server. The most likely classes will be load config and load result classes in com.sencha.gxt.data.shared. If you dont use them, it should not be necessary to copy it.