However, I do not want half-hearts. I just want the health system to be a simple trio, and each time the player is hit they lose one heart image. I honestly don’t even know how or where the current code is accessing the half-heart images. Otherwise, I would just replace those images.
Without seeing the rest of the code… There will be Sprite variables heartFull, heartHalf and heartEmpty. (Note your case 1: has misspelt heartFull, the compiler should pick this up so assuming it is a typo here)
Delete the Sprite heartHalf; variable declaration
I will take a punt that this is a monobehaviour script and they are public variables so you should be able to see the names (heartFull etc) on the game object that the script is attached to; you can just drag a sprite there if you need to change the heart images.
I assume you have just used someone elses existing code for this?
Best bet is start with the basics; Unity Learn