i am in 3rd year of BE in electronics,want to do MBA but need a job before it through campus...what to do?how to prepare for both?

HI, It is a good idea. After completing your Btech you can go for atleast 2 year work experience which shall even be a plus point when you plan to do MBA further.
So at present you can focus on academic performances because when  you plan to do MBA further the 12th/btech percentage keeps importance.
Side by side you can gather information about MBA exams and the syllabus and sections under it.
You can easily start with English and GK from now so that later you are left with the remaining.
Hope this guides you.
All the best.