Your very first comparison
Code:
if (choice1==="piedra"&&choice2=== "tijeras"){return alert("GANASTE");}
else{return alert("PERDISTE!");}
means: Only return "GANASTE", if choice1 is piedra and choice2 is tijeras, in EVERY OTHER CASE it will return "PERDISTE" and will never do the other checks.
Try this
Code:
if ((choice1==="piedra"&&choice2=== "tijeras")||(choice1==="papel"&&choice2=== "tijeras")||(choice1==="tijeras"&&choice2=== "piedra")){return alert("GANASTE");}
else{return alert("PERDISTE!");}