What could happen if I gave three checks by $4000 each one and there is not money in my bank account? One guy gave me $12000 as a pesonal loan, but there is not any proof about that. I was paying but now I'm unemployed and I can't pay him anymore. He has the checks. What can he does?

In the USA, he can file criminal charges for bad checks.

Each one, being well over $500, will probably be a felony, so that would make three felony counts of check fraud.

You could very well be in a great deal of trouble.

Did he by any chance agree to "hold" any of the checks for a period?