an employer is offering 3K EUR as signing bonus, I was told that it's taxable, so I want to calculate how much tax do I have to pay and how much will be the 3K after all tax deductions?
I live in Berlin, my tax class is III.
As @Tomtom correctly pointed out, German income tax is progressive, so it will depend on your total income.
However, your employer should be able to tell you how much they'll have to deduct and there's nothing wrong with asking them this. This will tell you how much you should expect to arrive at your account. You can also check that with the online calculator provided by the tax office, section Lohnsteuer (wage tax).
Your income tax declaration may lead to some adjustments later on - but it will probably be difficult to pull a precise unambiguous number for the bonus out of that other than doing a marginal calculation.
You can also use the preview function of the income tax declaration software to get an idea beforehand. That is only as good as you are in entering all the relevent information, though.