If you do not have a good academic performance previously, then there is nothing that you can do to change it. So there is no point in crying over that. Placement at college campuses is itself dependent on so many variables, including luck, that it is difficult to predict who will be placed where. People with stellar grades and superb academics throughout have struggled to land top jobs while average grade students have bagged dream roles in big companies such as Google and Apple.

Now there are certain things which are in your hand which can certainly help improve your chances during placements.

• Aim for better grades in whatever time you have at IIT Madras. 
• Do a good internship. Get some practical exposure to the industry. Remember internships are more about learning than getting good compensation.
• Work on research projects or industrial projects at the college level as well as industrial level, if possible. Also, aim for publishing your research in good journals. This will improve your credibility drastically.
• Most important of all, improve your communication skills, both verbal as well as written. This is a soft skill that is often neglected by students, but in fact, is one of the most important skills to possess and could be the x-factor of an average student over a strong candidate.

If you do all the above things you will definitely bolster your chances of getting selected at those companies. But remember one thing, you should aim for a job profile which interests and suits you over an uninteresting job at a top company.