You would only need a 3-piece coupling (also known as a union) on one end of the elbow. Here's why: Purpose of a 3-Piece Coupling/Union

The main purpose of a union is to allow you to disconnect and reconnect pipes without having to rotate them. This is crucial for situations where you need to install or remove components (like an elbow) in a fixed piping system.

Spinning the Elbow

You use the union to get one end of the elbow connected and tightened. Then you rotate the elbow to the desired position. After the elbow is in position, the other end of the elbow can be connected to another piece of pipe.

One is Sufficient

Having a union on only one end of the elbow provides the necessary flexibility to position and connect the elbow. Having unions on both ends would be redundant and unnecessary.

In summary: Use a 3-piece coupling/union on one* of the connections to the elbow to allow for rotation and positioning. Flag for review