rambo007 As indicated by the previous poster you do not have to use the same brand of device on all three TVs. However if your parents are new to streaming it would likely be easier to have all devices be the same brand. I have two TCL Roku Tvs, a Roku streaming stick 4k on two other dumb TVs, a Hisense TV using a google interface, two chromecast with google TV on two other TVs, and an apple streaming device on my main TV. As for using all of the TVs at the same time the answer is yes. I can stream on all eight of my TVs at the same time but most services like Netflix etc... limit the number of streams that can play at one time from that service. Also an app for a streaming service like Netflix can be installed on all of your devices whether they are the same brand or not. Perhaps just as important is what are your parents going to stream. Are they looking to replace cable TV with a streaming service? Are they going to continue to subscribe to their existing cable service? Are they looking only to stream from services like Netflix, Amazon, Hulu, Disney +, Apple TV, Peacock etc...?
My best advice when wanting to view on multiple platforms is to NOT subscribe to any premium sevices anywhere other than directly on each service's web page. They can use the login credentials they establish with each service on any and all versions of that service's app they install to any platform.
If they subscribe to premiums through third party services (Roku, Amazon, Apple, etc.) they may only be able to view that subscription via the specific third party through which they subscribed.
