int[] array1 = {1 ,2 ,3 ,-5 ,2 ,0 };
int[] array2 = {1 ,2 ,3 ,-1 ,5 ,0 };
int[] array3 = {1 ,2 ,3 ,-6 ,2 ,9 };

Console.WriteLine(array1.Union(array2.Union(array3)).Where((x) => x >=0).Count)

Console.WriteLine(array1.Union(array2).Union(array3).Count(x => x >= 0));

Console.WriteLine(string.Join(",", array1.Union(array2).Union(array3).Where(x => x >= 0)));
//Prints: 1, 2, 3, 0, 5, 9

array1.Union(array2).Union(array3)

var concatenated = new int[array1.Length + array2.Length + array3.Length];
array1.CopyTo(concatenated, 0);
array2.CopyTo(concatenated, array1.Length);
array3.CopyTo(concatenated, array1.Length + array2.Length);

var arr1 = new[] {1, 2, 3, -5, 2, 0};
        var arr2 = new[] {1, 2, 3, -1, 5, 0};
        var arr3 = new[] {1, 2, 3, -6, 2, 9};
        var result = arr1
            .Concat(arr2)
            .Concat(arr3)
            .Distinct()
            .Where(i => i >= 0)
            .ToArray();

//
    //
    ///<summary>Create a new array as concatenation of all given arrays.</summary>
    public static T[] Concatenate<T>( params T[][] args ) {
        if ( null == args ) return null;
        // Get argument lengths
        var count = args.Length;
        if ( 0 == count ) return null;
        var lengths = new int[ count ];
        // Compute all and total lengths
        var total = 0;
        for ( int i = 0; i < count; i++ ) {
            lengths[ i ] = null == args[ i ] ? 0 : args[ i ].Length;
            total += lengths[ i ];
        }
        if ( 1 > total ) return null;
        // Create target array
        T[] a = new T[ total ];
        // Copy all together
        total = 0;
        for ( int i = 0; i < count; i++ ) {
            if ( null != args[ i ] ) {
                args[ i ].CopyTo( a, total );
            }
            total += lengths[ i ];
        }
        return a;
    }