I agree with what Uncle Bob has already shared and let me add...
"La." indicates his "home of record" Louisianna (sp).
an RDM3 would be "Radioman 3rd Class", a 3rd class Petty Officer charged with either the operation of or maintenance of radio communications equipment aboard ship or on shore.So in terms of description "RDM3" is both his job title and his military rank.A 3rd Class Petty Officer was generally considered more than competent to operate with little supervision (such as the sole operator aboard a small craft) though not usually of high enough rank to supervise others.A search on the web of "naval enlisted rank" might make this clearer for you.Radiomen are sometimes nicknamed "Sparks" or "Sparky" due to the insignia of their specialty (rating mark) which looks like lightning bolts.
What you read as "USNF" is most likely supposed to read "USNR" United States Navy Reserve and in actual practice was (and still is) not an indicator of whether a veteran saw combatant service.Many reservists serve in combat during times of war -- a tradition begun as early as the American Revolution.The best way to clarify the actual service your uncle performed is to locate records of his service.Though they're available from the federal archives, let me suggest that you first visit your local county (Parrish) Veteran's Service Officer and see if he knows whether such records were recorded locally.He can also help you decipher all of the "militarese" language you will find in those files.
Another source of his service history might be found among old photos.All of the insignia that you see on a military uniform has a meaning -- if you can find a photo of him in uniform then maybe one of us old vets can help you decipher what it all means.
The reference "WWII" indicates that your Uncle served in the time period established for service during the Second World War -- it isn't an indication of whether he served in combat or even left the continental United States.It does help you establish though that he was in the Navy sometime between about 1941 and 1945.
Remember always that every single service that every single veteran performed for their country was magnificently important, from the lowest Seaman to the highest Admiral because without the one, there is no need for the other.