Three boards, six levels.
The same four skills, the same pass mark. The difference is what you have to repeat after a fail.
telc Deutsch
telc exams are passed or failed as a whole. The reading part also carries the Sprachbausteine, a pure grammar and vocabulary section the other boards do not have.
The exam is marked as a whole. Whoever fails repeats every part.
Goethe-Zertifikat
In the Goethe-Zertifikat every module counts on its own. Pass three and fail one, and you repeat only that one. That makes the exam dearer per module, and much cheaper when something goes wrong.
Each module counts on its own. A failed module can be repeated alone.
ÖSD Zertifikat
The ÖSD certificate comes from Austria and is recognised across the German-speaking world. Its tasks draw more often on everyday situations in Austria.
The exam is marked as a whole. Whoever fails repeats every part.