The Roku 3 seems like an awesome device, you can bet it won't have Showmax on it though..
This not an easy one to answer "simply" -
When you say "stream movies, series" - what would the source be? Your own collection (Via something like Plex or Kodi (was called XBMC before), or just showmax? Remember any device like Roku, Chromecast (Google), Amazon stick etc essentially run some applications on them - which makes your tv "smart" - for example it will have a PLex app, or Netflix etc - but to access your movies - but you need a media server in the back - so it's not a simple answer. Media servers for the non "geek" is a lot of work, there is no easy solution
Your TV would definately have HDMI - unless it's REALLY old - so just connect a notebook/small desktop to it - and watch your Showmax that way. If you really don't like wires - then you get a few "hdmi over wifi" type dongles that you could give a try - but a 10M HDMI cable at hificorp is much simpler, and cheaper..
Perhaps it's easier to replace the TV with one of the many specials everyday - some of the local Samsungs etc might even have a showmax app - but check with them.
Had a quick look on the Showmax site: (
https://www.showmax.com/eng/help/smart-tv/1)
Which Smart TV’s are supported?
Samsung Smart TVs (2012 and later)
Samsung Tizen Smart TVs (2015 and later)
LG NetCast Smart TVs (2012 to 2014)
LG WebOS Smart TVs (2014 and later)
Good luck!