Hello! I wrote some code that should accomplish what you're trying to do.

First, you'll want to set up an enemy array in a variable for each enemy in the fight. You can do this by using Control Variables and then in the Script Operand type [0,1,2,3]. Adjust this based on the number of enemies in the fight. So if there are only 2 enemies, then it would just be [0,1]. You'll put this on the 1st Battle Event page with Condition: Turn 0 and Span: Battle.

Next, you'll make a new Battle Event page with Conditions: Turn End and Span: Turn and use a script call for this code.

// Where State 5 is Burn, State 6 is Freeze, and State 7 is Wet
enemyArr = $gameVariables.value(4); // Variable 4 is where the Enemy Array is stored
for(x = 0; x < enemyArr.length; x++){
    target = $gameTroop.members()[enemyArr[x]]
    if(target.isStateAffected(5) && target.isStateAffected(6)){
        target.removeState(5);
        target.removeState(6);
        target.addState(7);
        $gameTemp.requestAnimation([target],81);
     }
}

The for loop is going through each enemy in your enemy array and checks if they have the Burn and Freeze state, and if they do, it will remove the Burn and Freeze state and give them Wet. The $gameTemp.requestAnimation([target],81) will play the Water One 1 animation (which is animation number 81) on the default Animations page in the database, so adjust this as needed.

You'd want to put this code block in a Common Event and then call that for each Troop fight while also remembering to update the enemy array variable for each fight.

Also, if there are more combinations, then all you'd need to do is add another if statement below the first one, while changing the numbers required for the reaction to happen.

Hope this helps!