Concentrated $\mathit{HCl},\mathit{ HNO}$₃, and a 3:1 combination of concentrated $\mathit{HCl}$ and $\mathit{HNO}$₃ were taken in separate tubes marked A, B, and C, respectively. In each test tube, a little bit of metal was placed. In test tubes A and B, nothing changed; nevertheless, the metal dissolved in test tube C. The metal could be __________________.

The metal could be Au, Pt.
According to the question, the mixture to test the metals in test tubes A, B, and C is aqua regia. Gold(Au) and platinum(Pt) can be dissolved using a solution of three parts concentrated hydrochloric acid and one part concentrated nitric acid (Aqua regia - 3:1 ratio).
Platinum(Pt) and gold(Au) can only dissolve in aqua regia; they cannot dissolve in weak acids.