When applying for a professorship (W1/W2/W3) in Germany one always has to list three and sometimes more previous employers/supervisors who are willing to write a reference letter. At which stage of the application process are these people ususally contacted and asked to write this reference letter? Before the applicants are invited, after the invitations are sent or after the visits/talks of the applicants?
I've never heard of the custom of requiring the candidate to provide a list of references for a faculty position in Germany; the hiring process is very different from the US and UK. The procedure is usually as follows (there's bound to be a more complete answer on this site, so I'm summarizing):
The hiring committee meets and decides on a text for the announcement as well as on criteria by which applicants are excluded (wrong subfield, no service teaching experience, that sort of thing).
After the deadline, the hiring committee meets again and selects (usually) four to eight candidates for a formal interview.
After the interviews are completed, the hiring committee meets again and decides which of the candidates to keep in consideration. It is for these candidates (usually three or four) that reference letters are requested -- but in a specific fashion: Two or three eminent experts in the subfield are selected by the committee (often beforehand, i.e., in step 1) who are then asked to review and rank all candidates. Needless to say, previous employers or supervisors are not the best candidates for this (but sometimes can't be avoided).
Based on the reference letters and their own opinions from the interviews, the committee decides on a ranked list of candidates to be offered the position. Then the administration takes over...
Note: I'm going by my own field -- mathematics -- here, details and especially the numbers may differ in other fields. But from what I've seen of the process in other fields, the general procedure is the same. If individual reference letters are indeed required in addition to the comparative ones, I'd assume they are requested as late as possible -- and only if necessary -- to avoid needless work for the writer. (There is significantly less of a "reference letter industry" in German academia.)