Latest stable version is WildFly 9.0.1.Final you can grab it from http://www.wildfly.org/downloads/
JBoss AS project was renamed to WildFly after version 7.
As for migration goes, just try deploying to 9, and if you see any problems post on forums.
there are some migration docs https://docs.jboss.org/author/display/WFLY8/How+do+I+migrate+my+application+from+AS5+or+AS6+to+WildFly
but none are talk about 3.x to 9, which is really old.
There ware folks on forums that did it, so you can just search for the problems you find as you go and probably someone has already did it.