class Pair
{
    public Pair(int t, double v)
    {
        Time = t;
        Value = v;
    }

    public int Time;
    public double Value;
}

void Main()
{
    List<Pair> list1 = new() { new(1, 123), new(2, 234), new(3, 345) };
    List<Pair> list2 = new() { new(2, 567), new(3, 678), new(4, 456) };
    List<Pair> list3 = new() { new(1, 789), new(2, 890), new(3, 900) };

    var times1 = list1.Select(e => e.Time);
    var times2 = list2.Select(e => e.Time);
    var times3 = list3.Select(e => e.Time);
    var allTimes = times1.Concat(times2).Concat(times3).ToHashSet();

    var results = allTimes.Select(t =>
    {
        var value1 = list1.Find(x => x.Time == t)?.Value;
        var value2 = list2.Find(x => x.Time == t)?.Value;
        var value3 = list3.Find(x => x.Time == t)?.Value;

        return new { t, value1, value2, value3 };

    });

    results.Dump();
}