Hello dear

According to the eligibility criteria of BHU PET for applied microbiology you must need any 2 of the subjects- botany , zoology ,biotechnology, microbiology, chemistry, industrial microbiology, life sciences, environmental sciences with minimum 50% Mark's in aggregate in your degree

So as such you had zoology for 3 years and if u have minimum  50% you can choose this and in botany also if u had secured 50% then you can choose this as 2nd subject and then apply for it .

Hope its helpful

Good luck