a large loan to three parties --they say they can't repay

Well, unfortunately, its not terribly uncommon for parties to a contract to say they can’t pay when that money comes due.  However, you mention you have a contract with these guys so you should have a case for breach of contract.  Of course, this is coming upon the assumption that they aren’t paying in conjunction with the repayment procedures outlined in the contract.  For example, it’s a different story if the contract says they are to pay at X date and you call up asking for the money earlier than that date.  That said, if the money is due and they aren’t paying, it may be time to take the next step.  My advice would be to consult with a contract or business litigation attorney and have them litigate this for you.  Litigation can get pretty complicated so you don’t want to make any mistakes that will cause great delay, or, even worse, a loss.  Best of luck.