I checked the salary scale for a W3 professorship position in Germany. The annual income is about 75K Euro which seems pretty low for someone managing about 20 people. Is there any other payment beside the base salary? For example, some additional payment from DFG/EU projects or for supervising students?
As Dirk writes, W3 only gives the Grundgehalt (base salary), which varies by state. As a public servant, you will automatically get a few hundred EUR on top if you are married and/or have children (Familienzuschlag). Note also that in some states (Bayern, Hessen, Sachsen), your W2 or W3 Grundgehalt will currently increase in two steps after 5/12 or after 7/14 years.
You can supplement the Grundgehalt with various Zulagen. These are pay increases, not one-time bonuses. You can get Zulagen either for a set period (befristete Zulage, i.e., a temporary one), or for the rest of your working life (unbefristete Zulage).
You can negotiate so-called Leistungszulagen (a performance bonus), for instance if you manage to get lots of third-party money, publish a lot, or supervise a lot of dissertations, or similar things. You typically discuss multi-year targets with your dean and/or university president or chancellor.
Some German states (Brandenburg, Bremen, Hamburg) currently give you a non-trivial Grundleistungszulage without any preconditions.
You may get Funktionszulagen if you hold particular offices, like being the dean or managing a larger group, or managing a specific institution or institute.
If you get an offer from a different university, you will start haggling with both the new university and the one you are at. Both usually want you, so both will offer you a Berufungszulage (a "job offer pay increase", at the university that extended the offer), or a Bleibezulage (a "staying pay increase", at the university that tries to keep you). In contrast to industry, this kind of haggling is normal. (In industry, if you get an outside offer but stay in exchange for a higher salary, your manager will usually assume that you won't stay much longer.)
As a brand-new professor, you may be able to get a small initial Berufungszulage, but most other Zulagen will only be negotiatable after a few years.
The Deutscher Hochschulverband is a good place to look for additional information. This is basically a union for professors and almost-professors in Germany, and they offer coaching to their members. Very much recommended in your first (and subsequent) salary negotiations - they know what you can reasonably demand and what not. A membership is definitely worth the dues. They also publish the pay scales currently here, in German, although this link will likely rot over time.
In addition, you can get additional money from outside sources:
You can have consulting contracts in industry. You will usually need to clear this with your university, and your outside commitments will likely be capped at 8 hours per week or similar, but this can of course still mean serious money.
If you are a professor at a medical school or a Universitätsklinik, you will be able to see private patients and take home some of their payments, although this has been getting less lucrative in recent years, with the clinic keeping a larger cut from younger professors. You could set up your own practice on the side, but this would again be subject to limitations set by your employer.
As Wrzlprmft notes, you can get a little money out of the Verwertungsgesellschaft Wort, or VG Wort - see some information in English here. It essentially disburses monies as payments for people using your copyrighted works (they get their money from various sources). It's a very German institution. This will usually not be a lot of money, especially if you publish with a lot of coauthors. Here is what Academia.SE knows of VG Wort.
However, if you do publish a book, you will be able to keep any payments from the publisher. This will again usually not be a lot, and it's not specific to Germany.
Finally, note that a professor in Germany is a Beamter (public servant). This means that you pay far less in social security contributions, since you by definition can't lose your job (so no joblessness insurance contributions), and the state will pay your pension (so you don't need to contribute - although saving for your old age is still a good idea). The state also pays for part of your medical bills and those of your dependents (Beihilfe), and since you are a good risk (see above on not losing your job), you will get good deals on remaining health and other insurance policies. Bottom line: out of 75k EUR, you will take home a larger fraction than if you earned the same amount in industry.
No, in general there isn't. The catch is, that the salary you quote is the base salary and this is negotiable. The actual salary can be considerably higher.
If you are a post doc and get your very first offer, then it may happen that your offer will be the base salary. However, if you manage to build up a group of 20 people that you supervise then there will be no problem to negotiate a higher salary. Exact numbers on how high the salary can be are hard to come by (Germans tend to not talk about salary too much and professor's salaries are usually highly confidential). I don't know a an upper limit of the salary both for W3 or W2 but I know that raises can be that high that some W2 professor makes more than another W3 professor. A monthly raise of a few hundred euro is not rare and I suspect that a raise higher than a thousand euro is also not rare. Here is some article that quotes average W3 salaries between 6.300€ (about 75k€/year) and 7.500€ (about 90k€/year) (depending on the state) which indicates that there are still higher salaries. This article says even a raise higher that 5.900€/month is possible (i.e. about 100% on top).
Moreover, as a professor in Germany you may have "Nebeneinkünfte" but I can't say too much about that…