You will need a sale agreement where you will reallocate your share amongst the 3 of you. You should also sign an operating agreement now so that the rules are clear. You guys will need to use some reasonable way of valuing the business, so that you can use the valuation to determine the third partners cost to enter.

Step 1: get a lawyer. Your country/state/province will have specific ways to do this that nobody on reddit can tell you about from a one-sentence post.

Business broker here.

• Determine the value of the business.
• Have the new partner pay a 1/3 of the value. The payment can either go to the other partners or to the business itself depending on several factors.
• If the partner does not have the money to buy into the business, consider a payment plan, or define what the partner will contribute to earn the equity.
• Have your attorney write an operating agreement to define how the partnership will function, and a buy-sell agreement defining how the partnership will end.
• Consider setting up two levels of compensation to partners. Distributions are paid to all partners proportional to their share of equity, and salaries are paid to active managers of the business.

The operating agreement you created when you formed the business will detail who has what percentage of the business.

You’ll need to adjust that and possibly some Fed/State/County registration documents.

How are you currently set up? If you have a legal partnership, you will have to dissolve that partnership and reform, preferably under an LLC or SCorp.

Get the assistance of a lawyer, Enrolled Agent, or CPA.