Clint, Bruce and Tony had always felt incomplete. They had not found their soul mates until very later in life. They were a trinity and it took the almost end of the world to find each other and unite.
Bruce is the oldest. He gets his soul marks at 2 years of age. It is a blue dot near his chest and an arrow running down his later he figures out the dot is an arc reactor and his other soul mate is an archer.
Tony is second. He is born with one soul mark. A human DNA with a thread of green. He gets the second one 3 years later. It is a purple arrow running across his back. Years later he becomes a fan of the Hulk and meets his adorable archer.
Clint is the youngest and he is born with both his soul marks. It makes him happy, that there are two people who would love him. Years later he sits between a genius, the smartest man in the world and the man who turns into the strongest beast on the planet. He is the youngest and his soul mates protect him. He feels safe as he never did during his childhood.
These three are broken in their own way, but with each other they are complete.
They are a trinity.