Is zakat obligatory on money collected in an "association" for three years that has not been زكّ عنه (had zakat paid on it), and should zakat for past years be paid when the money is received or from other funds?

Zakat on wealth must be paid immediately once it reaches the nisab, because delaying its payment is a sin and a debt upon one's conscience. One must hasten to pay the Zakat for past years. Wealth used for trade is subject to Zakat, and the profit generated from it is zakatable along with the principal wealth.