Yes, if you accept a college in MCC round 3 and haven't joined any college in previous rounds of NEET counselling (including round 1 where you weren't allotted anything in round 2), your security deposit will be refunded to you as you did not accept a seat in the earlier rounds; the Medical Counselling Committee (MCC) typically refunds the security deposit to candidates who do not take up a seat allotted to them during the counselling process.