Hi I made a video about the problem I am having with my Action 3.
I cannot get any Sd card to work correctly in the camera.
I have tried x2 Samung Pro Plus V30, x4 Sandisk Extreme Pro V30 and still the camera throws up errors.
Sometimes it says NO SD CARD (when there is a card installed) then it will record for a few moments and then say SD CARD initializing.
All SD CARDs have been checked for errors and tested 100% fine. They also work fine in InstaOne 360 & GoPro.
This was a replacement unit from DJI after my first A3 came with focus issues, this one has worked for 4 weeks and is now failing.
Really disappointed in the quality of this product.
Here is the problem:

Man, that sucks! I know how irritating it can be when you buy a new product and you have to keep sending it back for repair or replacement because it keeps failing with various issues. Hope it will get sorted out.

For me it seems like it have problem with the internal reader.
Get it replaced

Unfortunately, it is time to return that replacement unit for another replacement unit that will hopefully be error free.  

Hi, SLiWooDy. Thanks for reaching out, and we're sorry for the inconvenience. Have you tried formatting the SD cards? If not, please do so. If it's done and the error remains, we're afraid you may have to consider sending the device back for further checking. You may submit a repair request at (repair.dji.com) or contact our Customer Support at (support.dji.com). Please reach us back here should you have other concerns. Thank you.