If they lost the Check the Money must still be in you account and you write them a new check. Problem solved.
Brian hit the nail on the head. If the check was lost and not cashed, then just write a new one and cancel the old ones. If the check was cashed, go to your bank (if you can not do this on-line) and get a copy of the cleared check. Bring this to the management office (always have a copy for yourself) as proof. This is the 3rd time that your landlord or management company has lost your check? I'd also question on whether I wanted to live there once my lease terms were met. If you do end up going to court, bring copies of your checks, cleared checks, bank statements, etc. Anything that proves that you paid the rent. However, if the checks never cleared...start keeping an eye on your bank account once you write a check and bring your account up-to-date with your lease (if those checks never cleared, then the money should still be there). Just as an FYI - evictions do not look good on a potential tenant when they are applying for a new rental home and yes, people do check this out along with background checks that will bring up any proceeding. You will always need a place to live, try to work this out.