It depends on what you want to know or measure. If you only need the data of the three phases, three converters are sufficient. But if you also want the leakage current, for example, the fourth is needed.

By the way, the manufacturer has quite good documentation on its products.
https://kb.shelly.cloud/knowledge-...

Anyway it seems not available an additional transformer to order by digitec. Am I right ? If NOT what is the article number to order ?