Triggering Record Creation on another Table

I am looking to create a formula to achieve the following scenario:

I have one table called clients - self-explanatory list of clients

I have one table called items

I have one table called descriptions. This table references both of the previous tables and stores a record for each item for each client.

What I want to achieve, is when i create a record in items, that it creates a record for each client in the descriptions table.

i.e. if I add Item #5 to the item table, I want to trigger a record creating in the description table for client 1 | item 5, client 2 | item 5, client 3 | item 5

I am assuming that I could create a formular to trigger on create/update.

Can someone assist me on what this formular would look like