There are 3 authors in a research paper. One of the co-authors wishes to withdraw from the study. Are there any forms that need to filled up? A form stating that the co-author has no say or hold or ang whatsoever to the paper anymore. He/she would literally give up all his/her rights to the paper.
Normally the answer would be no. A letter from them stating their withdrawal would be sufficient. However, it is possible that there is some interaction with a funding agency where they are already named and the agency requires notification.
It complicates the work, of course, since their intellectual contribution to date needs to be properly acknowledged. It may require rewriting any paper(s) to cite their work. Otherwise plagiarism issues might arise.