Yes, you can potentially change your course from Diploma in Mechanical Engineering to Diploma in Automobile Engineering after your 3rd semester. However, this will depend on several factors:

1. College Policy:

• Check with your college's academic department to see if they allow course transfers or lateral entry into other programs.
• Some colleges may have specific rules and procedures for course changes, including required credits, minimum GPA, and availability of seats in the desired course.

2. Curriculum Overlap:

• Assess the overlap between the two courses. If there are significant differences in the core subjects, you might need to take additional courses or clear backlogs.
• Consult with your academic advisor to understand the potential challenges and additional workload involved.

3. Practical Considerations:

• Consider the practical implications of switching courses, such as adjusting to a new curriculum, laboratory sessions, and project work.
• Evaluate whether you have the necessary aptitude and interest in the subjects related to automobile engineering.

Steps to Take:

1. Consult with College Authorities: Discuss your intention to change your course with your college's academic advisor or head of the department.
2. Understand the Transfer Process: Inquire about the specific procedures and deadlines for transferring to the automobile engineering program.
3. Seek Academic Counseling: Consult with academic counselors to get guidance on course selection, workload, and potential challenges.
4. Prepare for Additional Work: Be prepared to put in extra effort to catch up with the new curriculum and complete any required coursework.

Remember, the feasibility of switching courses will depend on your college's policies, your academic performance, and your personal circumstances. It's advisable to consult with your college authorities and academic advisors to get the most accurate and up-to-date information.