A while ago I wrote about the Trinity.
I gave an example of a family of three.
But here's a better example: a business partnership.
By Australian law, the partners in a partnership are 'jointly and severally' responsible for every action and liability of the partnership. If partner 1 takes a loan for the business, the creditor can go to partner 2 for repayment.
Dealing with any partner is dealing with the partnership. They are all of 'one mind' and one responsibility. You can talk to any of them and you have talked to all of them.
Now, amplify the intensity of that and imagine each partner has identical motives and objectives to the others, and they each are fully engaged with each other.
You have, then, the Trinity.