Extensions.cs

public static class Extensions
{
    public static Dictionary<TKeyOuter, Dictionary<TKeyMiddle, Dictionary<TKeyInner, TValue>>> ToThreeLevelDictionary<TKeyOuter, TKeyMiddle, TKeyInner, TValue, TInput>(
        this IEnumerable<TInput> input,
        Func<TInput, TKeyOuter> outerKeySelector,
        Func<TInput, TKeyMiddle> middleKeySelector,
        Func<TInput, TKeyInner> innerKeySelector,
        Func<TInput, TValue> valueSelector)
    {
        return input
            .ToLookup(outerKeySelector)
            .ToDictionaryWithDuplicateKeyCheck(
                x => x.Key,
                x => x.ToTwoLevelDictionary(middleKeySelector, innerKeySelector, valueSelector));
    }

    public static Dictionary<TKeyOuter, Dictionary<TKeyInner, TValue>> ToTwoLevelDictionary<TKeyOuter, TKeyInner, TValue, TInput>(
        this IEnumerable<TInput> input,
        Func<TInput, TKeyOuter> outerKeySelector,
        Func<TInput, TKeyInner> innerKeySelector,
        Func<TInput, TValue> valueSelector)
    {
        return input
            .ToLookup(outerKeySelector)
            .ToDictionaryWithDuplicateKeyCheck(
                x => x.Key,
                x => x.ToDictionaryWithDuplicateKeyCheck(
                        innerKeySelector,
                        valueSelector));
    }
}