Do I have to get a converter to send the signal from 3rd Gen Apple TV to an older Plasma monitor without HDMI inputs or will a simple adapter work?
Welcome to the Apple Community.
DVI may work, providing it supports HDCP, but is not officially supported. DVI doesn't carry audio, so you'll need some way of getting audio to your TV or speakers too.
There are some HDMI to component video adapters that “always work"
Retrieving data ...