The packages that fall under the servlet API are in the J2EE package - and have never been in J2SE. If you only use the servlet API, and not any other J2EE APIs, I would recommend just adding the servlet.jar (which comes with Tomcat, or other servlet containers) to your CLASSPATH, rather than downloading the whole J2EE.
As a note, if you are upgrading, I would skip 1.3 and go to 1.4 - 1.3 is quite old now, and 1.5 even will be released soon !