"According to the 2017 survey of the students who have graduated from the University for 3 years, about 80 percent are in employment, while the other 20 percent are mostly preparing exams for advanced studies or government jobs."
1. Those who have graduated can still be called "students" in the first half of the paragraph? If not, how should I rephrase it?
(Yes, as you have used "have graduated", it is understood that these people who are graduates NOW were students BEFORE they "have graduated". The use of present perfect tense describes something that took place in the past and the effect of the event is still ongoing now. Another example: "The people who have perished are honoured as war heroes."
In this second sentence, it is true that these people no longer exist, but just before they were killed, they were people who were alive. The use of "have perished" connects their death during the war to the present moment when they are honoured.)
2. Should I use past tense "were" instead when describing the survey result?
(These people graduated in 2014, so three years later, a survey was conducted in 2017 which was still in the past (it is now 2018). The results (80% employed versus 20% unemployed for various reasons) were describing a situation in the past; therefore, the past tense should be used. Example: "..., about 80 percent [of respondents were] in employment, while the other 20 percent [were] mostly preparing exams for...."