See work below.
Work Step by Step
Put a small amount of each metal powder in three separate test tubes. Add 1 M HNO$_3$ solution (1 mL at a time) until two of the metals have completely dissolved. HNO$_3$ will dissolve both Zn and Pb, but will not dissolve Pt. To the two test tubes containing dissolved metal, add a few mL of 1 M Ni(NO$_3$)$_2$ solution. Zn is more reactive than Ni and will replace it in the compound Ni(NO$_3$)$_2$, forming black Ni metal which should be clearly visible. Pb is less reactive than Ni so it will not react with Ni(NO$_3$)$_2$.