Here is mine. It has also null check.

public static void plusMinus(List arr) { double positiveQuantity, negativeQuantity, zeroQuantity = 0;

    foreach(var item in arr){
        if(item > 0) positiveQuantity++;
        else if(item < 0) negativeQuantity++;
        else zeroQuantity++;
    }

    int length = arr.Count;

    Console.WriteLine(positiveQuantity>0?(positiveQuantity/length):0);
    Console.WriteLine(negativeQuantity>0?(negativeQuantity/length):0);
    Console.WriteLine(zeroQuantity>0?(zeroQuantity/length):0);
}