The 3rd gen has a limited set of preset 'channels' that are sent via server to the device. It has been disctonued for sometime so you would not find it in any retailer and would need to resort to the used market. Also worth noting that newer services are opting to not support the server model that the 3rd gen uses (i.e. Disney Plus) and existing providers have also been dropping support (the latest was HBO I believe).
Both the Apple TV HD (4th gen) and 4K (5th gen) are more in line with the other products, where there is an app store and they run a proper operating system (TVOS). If you don't have a 4K TV then you won't be able to receive 4K video nor HDR, however if you think you may plan to upgrade at some point then it's still worth factoring in. Otherwise, the HD model would be the better option. I will leave a link so you can see more details on other features.
Apple TV 4K - Apple (CA)