In here 研究生包括 both Master's degree or doctoral degree students, hence precise translation should be

Master's degree or doctoral degree students in their third year of study

This was pointed out by another translator who replied to your question; however, I thought I would add my own answer as well. The term 研究生 refers to both masters and doctorate level students and their respective course of studies in the original Chinese sentence. Assuming that the three-year course of study holds true for students in both masters and doctorate programs, then an acceptable translation of "三年级研究生" would be "third-year master's students" or "third-year doctorate students". It is the case that doctorate level students are also referred to as "Ph.D. candidates", offering you an additional translation option. You can also rearrange the translation so that it reads as follows: "masters/doctorate/graduate students in their third year of study". I would encourage this latter option because your intent is to express in English information about how courses of study at the masters and doctorate level are structured. I did find some potentially useful examples in the Corpus of Contemporary American English that may help. I found these examples by searching for "graduate students" and arranging the KWIC results to the left.