I am currently collaborating with three researchers on three non-overlapping and independent research projects/papers (i.e. one with each). I have been asked to review a paper for a journal, for which the same three researchers are the authors.
I am tempted to take it and try to do an objective review, but I wonder if this would represent a conflict of interest. Of course, as they are constant collaborators, I am not sure if there would be any unconscious bias.