I gave realtor 3K down pyment signed a contingincy offer, seller accepted, if I the buyer back out,and cancel the check I gave today what happens?

I would not stop payment on check as that can be regarded as bad faith, however, some buyers do that and nothing happens.  The seller may willingly terminate the contract and consent to the return of your deposit, or as you suggest, if you determine that there are legitimate inspection problems or are unable to qualify for the loan under the terms of a contingency for financing, then there should not be a problem terminating the contract.  Sometimes there are incomplete or inaccurate disclosures prior to the signing of the contract that can serve as grounds for termination as well. This is specific to Florida law and does not constitute legal advice as the facts presented are anonymous and incomplete. This is intended for general education only and does not create an attorney-client relationship. This should not be relied on and you must seek your own attorney client relationship.