I am a law student (BALLB). I am interested in Forensic science too. Should I again spend 3 more years for Bsc or is there any alternative?

If you are interested in forensic Science, you can consider pursuing a Master's degree in Forensic Science or a related field after completing your law degree. Pursuing a Master's degree will take less time than completing another Bachelor's degree, and it will also give you a more advanced and spec