You can download it at:http://developer.java.sun.com/developer/earlyAccess/java3D/index.html
I'm not sure what is the difference between 1.3 and 1.3.1 (bug fixes I assume).
I downloaded/installed (OpenGL version) it and it seemed to run fine with Windows 2000 SP3, TNT1, 512M memory, 450 Mhz machine with JDK 1.4.1_01. Now when will JDK 1.4.2 be out? Supposedly it performs better in some benchmarks than 1.4.1_01.
Anyway, good to see that Sun is still bugfixing/enhancing Java3D. Now I'm looking forward to M6 of X3D!