Hello.
If you fail in 3 subject in HSC Board then there are two options for you either you can apply for supplementary exam and clear your subject or you can reappear for the whole class again and can have  a marksheet of pass for it , most probably forms for supplementary exams for HSC board opens in July now it depends upon the condition and your result comes probably till October so if you go for supplementary exam then again your 6 months are going to be waste and you will have to take admission for  next session in college.
Best of luck.