Answer:
Amos/Butch
Step-by-step explanation:
Since Amos and Butch are sure shots, Cody hits only 50% of the time, and each of them have to fire shots, then the order of shots can be that:
Cody shot Amos and missed, Butch shot Cody and hit his target and Amos shot butch and also hit his target with Amos left standing
Or
It could also have been possible that Cody shot Butch and missed his target, Amos shot Cody and hit his target, Butch shot Amos and hit his target with Butch left standing
Hence Amos/Butch have the highest chance of survival.