Issue with Purchased Course
I understand that you have purchased a 3rd class course on EduRev, but only the math subject is opening while the other subjects are not accessible. This can be frustrating as you have paid for all the courses and expect to have access to them.

Contacting EduRev Support
To address this issue, I recommend reaching out to EduRev's customer support team. You can contact them through the app or their website to report the problem you are facing. They will be able to assist you in resolving the issue and ensuring that you have access to all the courses you have purchased.

EduRev App Contact Number
Unfortunately, I do not have the specific contact number for EduRev app customer support. However, you can easily find their contact information within the app or on their website. Look for the "Contact Us" or "Support" section where you can find the appropriate contact details to reach out to them.

Resolution
Once you have contacted EduRev's customer support team and reported the issue with the course access, they should be able to investigate the problem and provide you with a resolution. This may involve fixing the technical issue that is preventing you from accessing the other subjects in your purchased course.
I hope this helps you in resolving the issue and enjoying all the courses you have purchased on the EduRev platform. If you need any further assistance, do not hesitate to reach out to their customer support team.