Is Zakat obligatory on money that reached the Nisab for the first time after ten years, and am I sinful for unintentionally not paying Zakat during the past three years, and how do I calculate the Zakat for previous years?

You are not sinful for not paying zakat (definition) due to forgetfulness, as Allah Almighty says: "Our Lord, do not hold us accountable if we forget or make a mistake," and as the Prophet, peace and blessings be upon him, said: "Indeed, Allah Almighty has pardoned my Ummah for mistakes, forgetfulness, and what they are coerced into." However, zakat is not waived and must be paid immediately. The method for calculating it has been explained in previous fatwas.