I want to spawn a series of 20~ objects with of 3 different types spread randomly throughout the series yet it will only spawn 20 of one object type

Question

I have a section of code within a game in Unity which is meant to create a series of 20 objects with a 3 in 5 chance of generating a Basic Object (BO), a 1 in 5 chance of generating Special Object 1 (SO1) and another 1 in 5 chance of generating Special Object 2 (SO2). The objects should be generated in a random mix of the 3 types spread throughout the 20 object sequence. However instead of creating a 20 object sequence of a mixture of the objects it creates a 20 object sequence of 1 type of object with the above chances of the sequence being made completely of said objects. Here is my code.

 int x = 0;
    do
    {
        var r = new System.Random();
        int objectType = r.Next(1, 6);

        if (objectType < 4)
        {
            SpawnBO();
        }
        else if (objectType == 4)
        {
            SpawnSO1();
        }
        else if (objectType == 5)
        {
            SpawnSO2();
        }

        x++;
    } while (x < 20);

I have tried putting the x++; within each part of the if statement but I still get the same result. Any ideas on how I can get it to create a mixture of the objects instead?