Is the angry partner entitled to a share in the sale of the land, which all three of them knew about and worked on before the dispute? And is it permissible for the other two partners to engage in a new business (selling used cars) without the angry partner's knowledge?

As long as you are partners in the buying and selling of real estate, each partner is entitled to their share of the company's profits. It is not permissible to deny any partner their share when selling the land, because that constitutes betrayal. The Messenger of Allah, peace and blessings be upon him, said: "Allah says: 'I am the third of the two partners as long as one of them does not betray his companion. If he betrays him, I depart from between them.'" As for establishing a new partnership for selling used cars between two of the partners without the knowledge of the other, there is no harm in that.