Obligatory costs that comes regularly

With €3000/month before (and some assumptions: single, church member, public health insurance, etc.) you get €1920/month paid out after deductions. This includes taxes, health insurance (€285/month, the employer pays the same again) and all other compulsory social contributions.
The deducted amount of taxes is approximately correct, so unless you have unusual circumstances the adjustment after declaration will be minor.
€980/month rent is rather high - you could definitely save by renting unfurnished and buying your own stuff!
How much you have left at the end of the month depends laregly on your lifestyle and spending choices.

Hello Beppi,

Thank you for your answer.
It is clear that I should not have any other expenses except the ones deducted from my salary.
I was not sure.

I do not agree with you about a non-furbished apartment and buying my own stuff but it is a personal point of view as I do  not know if I will be staying long enough in Berlin.

I have more questions about job process by German companies but I will ask on a different thread.

As Beppi mentioned, having a furnished apartment in Germany usually means paying a lot more than the normal rent. In most situations, even for someone just staying a year, buying basic furniture at IKEA or getting it used can save a lot. Of course for someone just staying for a month or 2 it will probably not be worth the effort. But without knowing the size, condition and neighborhood of one's accommodation makes it impossible to estimate the premium one is paying for the furnishings.

It is also possible that a furnished apartment might be relatively reasonable if the charge for it has risen slower than other rents, which have been sky rocketing in most Germany cities in recent years. But just knowing one has a place to live from the start is a comfort. I would just suggest that if you stay longer; then try to find out what comparable places rent for and consider your options for the future. What people generally find out is that they basically pay say 300 – 400 Euros extra per month for a place containing furnishings that could be bought once for 500 – 600 Euros.

These days, a 980,- Euro furnished apartment with utilities included might be a pretty good deal if it is a nice place with say 50 to 60 square meters. If it is just a 30 - 40 m2  one room place in a run-down neighborhood with just a kitchenette in a corner and shabby furnishings then it is probably over-priced.

A big consideration in judging an apartment is the kitchen. Traditionally, apartments in Germany were rented with a bare room for the kitchen and one had to acquire the appliances and/or cabinets separately. If custom made, one would have to get the next renter to either buy the kitchen furnishings or take them with you. Having an already built in kitchen can save a lot of trouble if it is nice and one doesn't have to pay a lot extra for it. Over the years it has become more common that there are built in kitchens that belong to the apartment – avoiding this question of having to buy from the previous tenant or getting things built in.