It is the topic of your dissertation thesis which determines its complexity and thus the time necessary to complete. But even if you have been carefully and, with the help of your academic advisors, you managed to keep your work in a reasonable frame, you should calculate rather four than three years for your PhD. Why? After you have officially submitted your written thesis to the department via the examination office, both of your main supervisors will be asked to hand in a written report on your work and they are given some time to finalize their evaluations. Your dissertation committee (Prüfungskommission) has to be assembled. And before your viva can take place it is mandatory that the written thesis be laid open to the faculty at least for two weeks. Taking into account the writing time of your reviewers’ reports, adding to it the difficulty to find a convenient date for the viva with a committee that comprises at least four professors, it is not unlikely that a couple of months goes by between the date of submission and the day of your viva. You should add these months to your calculation of your costs of living.
Unfortunately, there is no central office in Germany to apply for a stipend. Even more so, German universities in general do not have central funds from which to grant stipends and PhD fellowships. However, there are many foundations and other organizations (so-called “Begabtenförderwerke”) in Germany which endow such grants.
Alternatively, you may apply to one of the numerous Junior Research Groups and Graduate Schools, which, funded mostly by the German Research Foundation (DFG), grant stipends for at least the core period of your dissertation. But, of course, success is uncertain, since the number of applicants and the number of stipends available is far from being equal.
In both cases, the “Begabtenförderwerk” and the Junior Research Group, deadlines for application have to be met. In case of success, your allowance will start with the beginning of either winter or summer term, in some cases – if you have been accepted by one of the Graduate Schools or Junior Research Groups – only once a year, generally in winter. You should consider these conditions for your financial plan.
Another way to meet your living expenses, of course, is to apply for a position as “Wissenschaftlicher Mitarbeiter / Wissenschaftliche Mitarbeiterin” at a research institute or to a third party funded research project. These positions frequently are designed to allow for further (academic) qualification and explicitly reserve a portion of your contractual working time for your dissertation. It is, nevertheless, not always easy to find a comfortable balance between your duties on the job and the time you need for your own research. Do not hesitate to raise this issue with your academic advisor if you feel overloaded with work and look for a suitable solution which saves your dissertation.