In total, all the students in the 3rd year of their program are advised by how many professors?
3rd year of their program refers to yearsInProgram = 'Year_3'; professors refers to p_id_dummy
SELECT COUNT(DISTINCT T1.p_id_dummy) FROM advisedBy AS T1 INNER JOIN person AS T2 ON T1.p_id = T2.p_id WHERE T2.yearsInProgram = 'Year_3'