Yes a HM3 is a third class petty officer. HM denotes that the person's rate (or job person is assigned) in this case it is a hospital corpsman. The Number following shows his pay grade. HM2 is a second class petty officer and HM1 first class petty officer. A third class petty officer is an E-4 which is the equivalent to a Marine Corporal which is also an E-4.

Storekeeper-Petty Officer Second Class

Petty Officer is abbreviated PO and the rank is usually included, 1st Class, 2nd Class, or 3rd Class Petty officer.

E-4 is a 3rd Class Petty Officer, or PO3

E-5 is a 2nd Class Petty Officer, or a PO2

E-6 is a 1st Class Petty Offcier, or a PO1

CPO is a Cheif Petty Officer or an E-7

SCPO is a Senior Cheif Petty Officer, or an E-8

MCPO is Master Cheif Petty Officer, or an E-9

MCPON is Master Cheif Petty Officer of the Navy also E-9

There were many more military positions in Colonial times. The go from, Crewman Apprentice, Crewman, Specialist, Petty Officer Second Class, Petty Officer First Class, Chief Petty Officer, Master Chief Petty Officer, Warrant Officer, Ensign, Junior Lieutenant, Lieutenant, Captain, Major, Colonel, Commander, Rear Admiral, and Admiral.

The 2nd class indicates that they are a 2nd Class Petty Officer. They hope to become a 1st Class Petty Officer some day. A Signalman is one that is responsible for signals on the ship. The flags, flashing lights and semaphore are their responsibility.