PythagoreanTriplet.cs

using System;
using System.Collections.Generic;

public static class PythagoreanTriplet
{
    public static IEnumerable<(int a, int b, int c)> TripletsWithSum(int sum)
    {
        for (int i = 0; i < sum; i++)
        {
            var thing = Triplet(i, sum);

            if (thing.HasValue)
                yield return thing.Value;
        }
    }

    public static (int a, int b, int c)? Triplet(int currentIterat, int sum)
    {
        int a = currentIterat;

        for (int b = a + 1; (a + b) <= sum; b++)
        {
            for (int c = b + 1; (a + b + c) <= sum; c++)
            {
                if (Matches(a, b, c, sum))
                    return (a, b, c);
            }
        }
        return null;
    }

    private static bool Matches(int a, int b, int c, int sum)
        => a + b + c == sum
        && Math.Pow(a, 2) + Math.Pow(b, 2) == Math.Pow(c, 2);        
}