I own 3 apple tv devices but only 2 are activated.
Then we’ll treat that as two. Only activated devices count.
What are their models? If not the same model, then which one gives issues? Are they all set up/registered with the same Apple ID account?
Identify your Apple TV model - Apple Support
Find your Apple TV model number - Apple Support
What are their versions system software? Hulu says they require tvOS 13 or later.
… will not let me open hulu even though I am a premium subscriber
Is this just about Hulu?
Which plan do you have? Hulu uses the word ‘premium’ only for their add-on channels (HBO, Showtime, etc.).
Plans and prices - Hulu Help Center
Subscribing through Apple has two tiers: Hulu VOD, and Hulu VOD “no ads”.
Subscribing through Hulu directly has many more options, including Live TV, bundles, and add-on services.