u/Civil-Signature4096

I have recently started watching the show, made it to season 3 so far. However, I am very confused about Finn. Can someone please explain his place in the family? At times it seems like he is a brother to Arthur, Tommy, and John, which is also what a Google search says. However, we also hear people frequently talk about the "three brothers" during the show such as the company being evenly divided into three shares, and in season 3 episode 4 (which I am in the middle of watching) they talk of splitting the profits evenly 3 ways for Arthur to leave the family. If anyone can please clarify this with as few future spoilers as possible would be much appreciated

(Edit) Thank you to those who responded for the clarification