Question

What where the three reasons stated by Portia that would have prevented Bassanio from parting with the ring ?

Answer 1

Rather than give the ring directly to Bassanio at the very end of the play? Antonio, in speaking to Portia, has just made another grand promise to prove the depth of his love for Bassanio: “I dare be bound again, / My soul upon the forfeit, that your lord / Will nevermore break faith advisedly.” He sounds as if he’s ready to start up another trial scene in a competition to show whose love for Bassanio is more intense, his or Portia’s. So when she immediately produces a ring and gives it to Antonio (“Then you shall be his surety [or guarantor]. Give him this, / And bid him keep it better than the other”), it deflates Antonio’s gesture and returns control of the matter to Portia.

Answer 2

The three reasons are;
If Bassanio knew what that ring meant in their marriage, if he know how my the woman who gave the ring to him was worth and if he knew the honor of promise that depended on keeping the ring, he wouldn't have given the ring away.