First of all if you are talking of a three year course,then it is a diploma course basically known as Polytechnic.It is mainly for the students who have passed 10th exams, because diploma and Higher Secondary has same preference.You can also do Computer Science and Engineering diploma after 12th Science.And you cay have good future in this.

Hopefully this solved your query.

For a Btech degree in computer science and engineering it takes 4 years. Of course you can try for this course after science stream in 12th. Try to score great marks in the entrance examinations for engineering to get admission in merit seat. You can also try for management seat but that's not advisable as it costs a lot according to different colleges.  

Hope this helps.