B.Tech Scope

If you stop your B.Tech civil engineering studies in 3rd year, all your efforts till date will go waste. You have an option of continuing your education and completing your B.Tech before seeking alternate career or seek alternate career now. If you share why you do not want to study civil engineering further and what you want to do now, somebody will able to advice you better. You may share further details for advice.

You can do either ACPDM or CCPD from IGNOU. Trust me these certificate courses are very good for you.
You can visit the following link for more details:
http://www.ignou.ac.in/ignou/studentzone/programmes/4
All the best.

M.Tech from a reputed institution is an asset in the acquisition of a job, in a reputed concern. It is not compulsory, but it is certainly an added qualification.

You should prepare for GATE and do M.Tech in a good college.

You have to qualify tests like GRE or TOEFL in order to pursue higher studies abroad. You have to apply separately for the colleges and universities. You may not get a scholarship to pursue your Master's degree abroad.

No, you don't need any work experience to pursue MBA. Candidates with work experience are preferred more as they have some first-hand knowledge of the field they want to specialise in. You should gain at least two years of work experience before pursuing MBA.

Masters in Mechatronics is the best course in mechanical engineering, mechatronics is the study of mechanical and electronics which helps to get the best job in a company.

For making a career in aviation industry after completing your B.Tech, you can go MBA in aviation management. There are many colleges that provide this specialization like:
IIM - all states : This college without a doubt is the best for MBA.
Other than this:
Andhra Pradesh Aviation academy
UPES, Dehradun
Narsee Monjee, Mumbai