Solution:

The best thing for you will be to shoot your first shot in air.



If you try to shoot at Jack and hits him by chance, Leo always hits on the target and you are dead.



If you choose to shoot at Leo and hits him by chance, then there is a fifty percent chance that Jack will hit you.



If you shoot in air, the next turn is of Jack and he knows that Leo is a better shooter which means that he will shoot at him. If he misses, he is definitely dead as Leo will shoot on him coz he shot at him. The next turn will be yours and you stand a 1/3rd chance of hitting him. Also if, he is able to hit Leo. The next chance is yours again and you can try the 1/3rd probability of hitting over Jack.



This is the best situation that you can face. At every other possible way, you will face a worse situation for sure.