According to Jython's documentation:

Jython is an implementation of the Python language for the Java platform. Jython 2.5 implements the same language as CPython 2.5, and nearly all of the Core Python standard library modules. (CPython is the C implementation of the Python language.) Jython 2.5 uses the same regression test suite as CPython, with some minor modifications.

Are there any plans to support Python 3? If so, when is it scheduled to be released?

Jython roadmap is definitely outdated.
However, on Frank Wierzbicki's Weblog (one of Jython's main developers) you can get an update, telling that Python 3 is definitely on the radar.

Unfortunately, it is not yet clear when, as it is stated in a comment in that same blog from 2010:

Jython will reach 2.6 and later a 3.x, but it's hard to give a solid time frame. I'm hoping to get a 2.6 out in a few months.

In an interview in 2017, Wierzbicki stated that Python 3 is desirable but difficult.

Langfristig wollen wir eine 3.x-Version veröffentlichen, aber das ist ein langwieriger Prozess, weil es eine große Menge an Veränderungen in der 3.x-Serie gab.

English:

We want to publish a version 3.x in the long run, but that is a drawn-out process, because there were a lot of changes in the 3.x series.

Jython – Python in der Java-Welt [Pirates of the JVM], JAXenter, 2017-04-12.