First, I’m not going to attack you for releasing the animal. I don’t release them myself, but I consider that personal choice. To me, these things are not as absolute as some people seem to think. And since you did not give details of how you released it, I have no idea what you did. And since I don’t know where you are, I don’t know the laws in your country or other legal jurisdictions.
I’m not sure how your electric fence is set up. Apparently there is a weakness where the possum got under it, through it, or over it. Since it came three different times it’s probably not a case where the power went out. That would be too much of a coincidence. As Owen said, look for that weakness. You may need another strand of hot wire or a physical barrier. Or a dumb question, have you tested them to assure that all strands are hot?
I don’t know what your coop or run look like. Do you lock them in the coop-only part at night or do you leave a door open? Possum are mostly nocturnal (I did see one feeding at my compost heap at 1:00 pm one bright sunny afternoon so mostly nocturnal does not mean always). Besides, if a possum can get in, so can many other things. Some of those can visit during the day. Still, nighttime is your time of highest danger from many things. It increases your safety level tremendously by securing them at night.
The more layers of defense you have the better. How would you set it up if you didn’t have that electric fence? I consider my run pretty predator resistant but there are ways a climbing predator could get in. But my coop is about as close to predator-proof as I can realistically get it. I also have an area outside protected by electric netting. Birds of prey can still get in but that has stopped practically everything else so far, other than snakes and rats. Is there a reasonable way you can strengthen you defenses other than the wire fence?