How is Zakat calculated on money invested in purchasing lands, building on them, and selling their units, knowing that profit and loss are accounted for after three years? And is Zakat due on these funds before the three years have passed?

Your share in the investment project is subject to zakat as trade goods. Therefore, you should appraise your share in the land and building at the completion of a full year (hawl) from the original capital. You then pay one-quarter of one-tenth (2.5%) of your total capital each year. Do not delay paying the zakat until the time of accounting after three years.