Helpers.cs

using System.Linq;

....

public static int SumOfMultiples(int max, params int[] possibleMultiples)
{
    int sum = 0;

    //Validate if the number is MultipleOf, then sum it
    for (int value = 1; value < max; value++)
        sum += possibleMultiples.Any(y => Helpers.IsMultipleOf(y, value)) ? value : 0;

    return sum;
}