Which of the following ranks has three chevrons and a diamond?

The rank that features three chevrons and a diamond is First Sergeant. In military insignia, the three chevrons represent the enlisted rank, while the diamond is indicative of the additional leadership position that First Sergeants hold within a unit. This rank is typically associated with greater responsibility, including duties related to the supervision and welfare of the soldiers in their unit.

Other ranks mentioned differ in their insignia. For example, the Master Sergeant has three chevrons but lacks the diamond, indicating it's a different level of leadership with distinct roles. The Sergeant Major, while also holding three chevrons, has a star or other insignia rather than a diamond, further differentiating its responsibilities at the senior enlisted level. Lastly, the 1st Lieutenant is an officer rank and does not involve chevrons at all, instead using a different insignia altogether to represent the officer corps. Therefore, First Sergeant is accurately characterized by having three chevrons and a diamond, signifying both rank and responsibility.