What is the test for granting immunity to a defense witness at risk of dismissal of the indictment?

In United States v. Burns, 684 F.2d 1066, 1077 (2d Cir.1982), cert. denied, 459 U.S. 1174, 103 S.Ct. 823, 74 L.Ed.2d 1019 (1983), we distilled from our prior cases a three-part test for requiring the government to grant defense witness immunity at the risk of dismissal of the indictment. First, the district court must find that the government has engaged in discriminatory use of immunity to gain a tactical advantage or, through its own overreaching, has forced the witness to invoke the fifth amendment. Second, the witness's testimony must be "material, exculpatory, and not cumulative". Third, the testimony must be unobtainable from any other source. We cited this three-part test approvingly in United States v. Calvente, 722 F.2d 1019, 1025 (2d Cir.1983), cert. denied, 471 U.S. 1021, 105 S.Ct. 2030, 85 L.Ed.2d 313 (1985) and, most recently, in United States v. Pinto, 850 F.2d at 935.