1)You can put in as much as you want but you cannot deduct more than
20% of your income or more than 6682.- in 2011
2) You will get taxed at the moment you take the money out of the 3a account. The tax will be calculated as follow (See § 37 of
Canton of Zurich Tax Law): Calculate the tax that you have to pay as income on 1/10 of the actual amount.
See this table. But it has to be at least 2%. Multiply this number with 10. Thats the share that goes to the canton. Now multiply it with the Steuerfuss of your commune. That goes, logically, to the commune. If you are registered with one of the official churches then they will get there share too. This will sum up to a tax of approx 4 to 5% for small amounts (around 300'000.-).
Play with this calculator to see it in action
3) Is it worth to pay extra into the 3a account? One has to calculate the the maybe better capital gain of a 3a account (2,15% WIR-Bank) over other forms of investment (1% Postfinance e-dposit, 0.73% Swiss Bonds, stock market) versus the payout tax. Do this as a homework.
4) Is it worth to have multiple 3a accounts? Yes, because you do not have to cash in all your 3a money in the same year you will pay a significantly lower overall tax. Do as a homework, calculate tax for one account of 500'000.- and two accounts of 250'000.- each.