Be sure and set up the Ant. You have to add .../bin to your path and set the ANT_HOME environment variable as the directory where Ant is installed (ex. .../apache-ant-1.6.2).
Note that J2SE SDK and Ant is necessary to build P3 and not necessary only to use P3. Only J2SE JRE (a subset of J2SE SDK) is required when binary distribution of P3 is provided and you get it.
You can either get source distribution or check out source code from CVS. See the instruction on the download page.
If you have source distribution of P3, unpack it and have the source tree of P3.
Once you have the source tree, change into the installation directory.
You are now ready to build P3:
Now you have compiled binaries in the dist directory, which did not exist before this build process.