How to make coloured tetris-like blocks during runtime

I have 3 game objects. They are all different block types. red block, blue block, and green block.

I want to make them spawn and fall like tetris pieces in sets of three. How do I do that? When I made a tetris game I used this. The difference is I made prefabs of the tetris pieces. This was easy. There's only like 7 pieces in tetris. I made a prefab out of the one game object called block by copying it multiple times into a parent game object on the scene, then dragging the parent game object into the project asset folder.

3 blocks and 3 colours is way more pieces. And if I add even 1 more colour it'll be impossible to do it this way realistically. How can i go about spawning my 3 game objects randomly in sets of three?

     //randomly selects a tetris piece
     int i = Random.Range (0,groups.Length);


     //spawn piece at current Position
     //transform.position is position of spawner. Quaternion identity is default rotation. Quaternion is the rotation handler
 

     Instantiate (groups [i], transform.position, Quaternion.identity);

I'm very new to this, so I don't know if I explained how I made the tetris thing properly.