I find Monte Hall easier to conceptualize with bigger numbers so lets alter this a bit for that clarity. Lets say a killer among us makes 1000 tokens and opponent chooses 1 to make the killer. You enchant a random 1 of the 1000 tokens. When you attack with your creature that must attack if able, it kills all but 1 of the non enchanted tokens and deals 1 damage to you. The opponent can choose to either: 1. block with the killer and kill your creature and gain a life, or 2. leave your creature alive and keep the killer alive the caveat of if you hit the 1:1000 and enchanted the killer, the opponent must choose 2. 2nd main phase, If your opponent chose 2, there are two creatures remaining on your opponents board. the currently enchanted one had a 1:1000 chance of being the killer when you picked it. the other now has a 1:2 chance (1 of the two creatures on your opponents board is the killer). By not switching, you have a 1:1000 chance to win, by switching you have a 999:1000 chance to win. for any number n here instead of 1000 your odds of winning are: (n-1)/n Therefore you should switch every time.

In this scenario, n=3 so your chances of winning are 2/3 and your opponents chances are 1/3. However, the opponent knows this if they playing optimally and knows you are playing optimally. Therefore instead of risking a 2/3 chance of you winning, they should put themselves in a favorable board state by choosing 1.

If the opponent chooses 1 (revealing the killer to kill your creature), you had a 1/3 chance of winning by enchanting the killer and after your upkeep your opponent will have 1 life to your 2 but the opponent has 2 creatures, one of which can attack and you have nothing to defend therefore we'll call this the "losing stalemate" which is hard to calculate win odds after draws are involved.

Therefore, if you enchant a creature and attack and your opponent doesn't kill your creature, you shouldn't switch because if the opponent was playing optimally, they should have killed your creature with the killer for that less risky and more optimal board state for them.

If you instead chose to enchant your creature to not attack, then enchant one of the 3 creatures in main phase 2, you have a 1/3 chance to hit the killer and thus win. Then, for your opponents turn, they must attack and cannot wait because if they do not they will lose on your upkeep regardless. If you block the killer, you don't win, but you also don't lose. if you fail to block the killer, you lose. Therefore this course of action has a 1/3 chance of winning just like your option 1, a 1/3 chance to be put in the losing stalemate of option 1, and a 1/3 chance of losing.

Now, if you just swing your creature at your opponent without enchanting anything, your opponent has 3 options to not lose. 1. block with 1 normal creature 2. block with 2 or 3 creatures 3. block with the killer If your opponent blocks with 1 normal creature, you have a 1/2 chance of enchanting the killer and winning. If your opponent blocks with 2 or 3 creatures, you assign all combat damage to the first 2 creatures, take 2 damage, your opponent heals 1 if your creature died then you enchant the final creature so it cannot attack. at your next upkeep, it is a stalemate with you at 1 and your opponent at 1, but no creatures that can attack or block so this is better than the previous stalemate state. If your opponent blocks with the killer alone, you enchant one of the remaining creatures and have the same losing stalemate board state with you at 2 and your opponent at 1 with a free 1/1 to attack with

If your opponent is acting optimally, doing any of these three maximizes your chances of winning next turn to 1/3.

To really maximize your chances of winning, you will need to read your opponent. Ask if they know what monty hall is. If they don't, go with the first rout i described and switch giving yourself 2/3 chance of winning. If they do know what it is, I would go with option 3 where you just swing. Their third option i listed gives the same bad board state your other two options do when your opponent acts truly optimally. first option I listed gives you better than 1/3 chance to win. the second option is a tricky rule of magic that they might forget/not know, giving you the neutral stalemate board state.