Generally, all the newer Roku devices need to be connected to a TV that has an HDMI port. Now, there are some older models like the Express+ 3710 and 3910 that had composite cables (those RCA plugs type that went into Video and Audio ports) that you can still find on Ebay and probably Amazon. They also sell HDMI to Composite or RCA converter boxes for use with older TVs as well that would make the newer Roku devices work with an older TV. Keep in mind, you need the converter box, not simply an HDMI to Composite/RCA adapter cable. (The cable adapter alone will not generally work). However, my Premiere does work with just an adapter cable so may be device specific which models require the converter box. Generally between $10-20 for simple converters. Since these are converters, there is some loss of video quality. More expensive converters will give you better quality. If you are simply using this third TV as an extra with one of the Express models or the Streaming Stick, then this may be a suitable route for you.
If you decide to go with the HDMI converter box setup mentioned above, then should be able to use any of the Roku devices available to you, be it the Streaming Stick, Express, Express 4k/4k+, Ultra, or Streambars. (The Streambars may be the most difficult to get working though with an older TV.)
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Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941