What is the amount on which zakat is due now: is it the full initial amount until its due date, or the currently remaining amount including what is due after 3 months, or the amount after full payment of the installment?

The zakat due date is determined by the passing of one year from the day the nisab (minimum taxable amount) was acquired, and it is permissible to pay it in advance. If you acquired the nisab and a year has passed while it is still in your possession, zakat becomes obligatory on it. Your intention to buy an apartment is not considered unless the transaction is actually completed. If Ramadan is the month in which you acquired the nisab, then zakat must be paid in Ramadan. If you buy the apartment before the year is complete, then you pay zakat on the remaining money. The amount expected after three months is deducted from the remaining amount, and you pay zakat on the rest, provided you have no other money. Otherwise, you pay zakat on the entire amount if it exceeds your basic needs.