DOM level 3 is not turned on by default as of 2.6.1
Download source (and tools) and use Ant target "jars-dom3":
%[xerces-2_6_1]$ source build.sh jars-dom3
Xerces-Java Build System
------------------------
Building with classpath /opt/java/j2sdk1.4.2_03/lib/tools.jar:/opt/java/j2sdk1.4.2_03/lib/classes.zip:./tools/ant.jar:./tools/xml-apis.jar:./tools/xercesImpl.jar:./tools/bin/xjavac.jar
Starting Ant...
Buildfile: build.xml
init:
[echo] ---------------- Xerces-J 2.6.1 [1999-2004] ---------------
prepare:
prepare-common:
prepare-src-dom3:
[copy] Copying 30 files to /home/tmp/xerces-2_6_1/build/src
compile-dom3:
[xjavac] Compiling 60 source files to /home/tmp/xerces-2_6_1/build/classes
[xjavac] Note: Some input files use or override a deprecated API.
[xjavac] Note: Recompile with -deprecation for details.
jar-dom3:
[jar] Building jar: /home/tmp/xerces-2_6_1/build/dom3-xercesImpl.jar
apijar-dom3:
[copy] Copying 1 file to /home/tmp/xerces-2_6_1/build
[jar] Updating jar: /home/tmp/xerces-2_6_1/build/dom3-xml-apis.jar
samples-dom3:
[copy] Copying 2 files to /home/tmp/xerces-2_6_1/build/samples
[xjavac] Compiling 4 source files to /home/tmp/xerces-2_6_1/build/classes
[xjavac] Note: /home/tmp/xerces-2_6_1/build/samples/dom/ASBuilder.java uses or overrides a deprecated API.
[xjavac] Note: Recompile with -deprecation for details.
sampjar-dom3:
[jar] Building jar: /home/tmp/xerces-2_6_1/build/xercesSamples.jar
jars-dom3:
BUILD SUCCESSFUL
Total time: 20 seconds
In Java 1.4, the resulting jars need to be copied to jre/lib/endorsed to override the default DOM2 implementation
When released Java 1.5 is supposed will probably support this by default