They may have not sent you a card that is actually compatible with the P3. Back when I bought my P2V+, it came from Drones Etc., and they sent me a 64 GB microSD. The V+ couldn't even accept a 64 GB card.
Use the SD card that came in the P3 for now. It's only 16 GB, but it's compatible. Check the card that Drones etc. sent you. It should have a speed rating of 10 (indicated be the number 10 inside a circle on the card). If the number is lower than that, then they sent you a card that isn't sufficient for use in the P3.