Applets using jnlp can be forced to use desired JRE using <j2se> element e.g. <j2se href="http://java.sun.com/products/autodl/j2se" version="1.4.2*" />.
This ables applet to use desired version of JRE but that's not true in case of plugin3. Applets always use the latest installed version(1.6.0_22 in this case)
1) Install plugin3 bundles and also install JRE 5u22 and 1.4.2_20
2) try to load following applets
http://sqeweb.sfbay.sun.com/deployment2/jitu/deployment_mustang_exec_ws/deployment/src/plugin/MultiJRE/MultiJRE/html/auto/MultiJREFamily142150Installed/JREMutliFamilyJNLP.html
http://sqeweb.sfbay.sun.com/deployment2/jitu/deployment_mustang_exec_ws/deployment/src/plugin/MultiJRE/MultiJRE/html/auto/MultiJRE142150SpecificInstalled/JREMutliFamilyJNLP.html
One of the applet should be loaded using 1.4.2_20, one with 1.5.0_22 and one with 6u22, but if instead all the applets are using 1.6.0_22 then bug is reproduced.
If Java console options is set to visible then 3 different Java consoles should get launched but that's not true as of now