The ender dragon will take damage when hitting a player in full thorns 3 netherite. This affects both mele as well as if the player stands in dragons breath (this one especially makes no sense).
The expected behavior is that the ender dragon does not take damage from thorns or at least can't take thorns damage when player is hurt by dragons breath.
Thank you for your report!
However, this issue has been closed as Invalid.
We're already working on achieving parity across versions where possible, and we're very keen to hear your requests or suggestions, so we'd encourage you to make a post at https://feedback.minecraft.net, or upvote an existing topic there.
If you're not sure whether your issue is a parity request or a bug, hopefully this article will help explain further: Parity Request Guide
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