I have recently appointed to be a reviewer for a congress in my specialization field. The problem that I got is which affiliation should I put? Actually I am working as a lecturer in two universities, part time, but also I am making my first year of Doctoral studies in a third one. I was thinking to put the last one, should that be correct? or should I talk with my supervisor first?
If you only adjunct at the other two institutions (my assumption based on saying you do it part time), I would list your affiliation with your doctoral program. Of course your CV should include these adjunct roles as well, they just aren't your main affiliation.
If you have developed a more intimate relationship with either the two institutions you teach part time at (like say you've been an adjunct for many years) you might consider listing them as an affiliation. Those part time adjunct gigs though don't typically support research activities that would result in outside roles such as a reviewer (for anything) or even being considered a specialist in the field.