As you want to get the details of so will you be able to participate in the placement programme after passing B.Tech and left one backlog in third year so here is the information of the same for you:

I want to tell you that there are only few companies which allows one of backlog

There are very few companies which place the students with backlog and give them certain period of time to clear that backlog.

But that depends upon your branch and college.

Also take guidance of training and placement officer of your college.