The summary, is somewhat inaccurate. Anotion does not give Bassanio 3thousands ducats. If he did, then there would be no play. Right? I think what you mean to say, is that Antonio went guarantor for the 3 thousand ducats that Shylock lent Bassanio. But shylock, instead of having his money returned, tricked Antonio into giving him "a pound of flesh' if he forfeits on the loan.