Yes, you can cancel your admission in the college where you got admission in the third allotment. However, there are a few things to keep in mind:

1. Cancellation Deadline: Each state has its own deadline for canceling admissions. You need to check the specific rules for KEAM (Kerala Engineering Architecture Medical) to find out the last date for canceling your admission.
2. Refund Policy: Some colleges may have a refund policy, while others may not. You should inquire about the refund policy of the college where you want to cancel your admission. In some cases, you may be required to pay a cancellation fee.
3. Seat Availability: If you cancel your admission in the third allotment, it will release the seat for other candidates. There is no guarantee that you will get admission in another college in the subsequent allotments.
4. Future Rounds: If you are not satisfied with the options available in the third allotment, you can wait for the subsequent allotments or rounds of counseling. However, the chances of getting your preferred college may decrease with each round.

It is important to carefully consider your options before making a decision. You may want to consult with a counselor or advisor to help you make an informed choice.

I hope this information helps you.