I am about to buy an Action 3. I have a Sandisk Extreme 1Tb card of the same specification as the 256Gb one. Can I really not use the card I have with the Action 3? What is like to be the problem if I do? Overheating or what? How much 4k 24fps can I get on a 256Gb card?

From what DJI says is that they support upto 256gb on this camera, however, thats not to say a 1tb will not work, it might well do, its just not supported. I doubt it will be overheating, possibly a not recognised issue. See this post, they use a 1tb, other has used a 512gb too https://forum.dji.com/forum.php?mod=viewthread&tid=297489#

Hi there. Thank you for reaching out. Sandisk Extreme 1TB MicroSD has not yet been tested and confirmed to work accordingly with Action 3 so we cannot guarantee the performance. I hope your co-forum members can share their insights using the said MicroSD. Have a great day.