E3 is not the "thing." "Getting rated" is where things get sticky. This is because depending on what you do, ratings advance differently. When I was in, the Navy "needed" electronics techs. So we advanced quickly. In fact "we" were called "push button PO's" a name obviously meant to be derogatory from those who claimed they "paid their dues." Well, the Navy made that decision, not us.
I was in for 6, and due to a paperwork error, my E4 was delayed. If not for that I could have been E6 at the time of discharge instead of E-5
There are several factors........Some commands aren't as helpful as others for advancement. If you are in a small command/ ship you won't
Plus, sometimes someone gets someone in their chain who either is not on the ball, or maybe worse, "has a hard on" for a sailor, and they won't be advanced quickly.
That's one reason I got out. One AH in your chain of command can delay a sailor for literally YEARS if they are thinking of a career.