I agree that 3 is a bit young to be getting over excited at not remembering letters. However if he does have a visial problem then it would be worth finding out about it now- as it may be able to be corrected and then not be a problem. There are all sorts of problems that can lead to dyslexia type vision- often to do with eye tracking (so moving the eye along a page). I'm not sure how theyll test though. other problems can be things like bleaching- this is when the letters "dispapear" into the page (think about tying to read something in really bright light). A lot of this type of problem can be corrected by wearing coloured lenses- called Irlen lenses. There is tuff about them on the web- may be worth doing a search in google.
However and this is a big however- I doubt the optometrist will check for this type of thing-they do do a test to see whether children can see a shape on a background of lines (I found it really hard!) - but mainly it's just a genral eye check.
MY ds1 has some sort of visual problem as he always shuts one eye when looking at something close up- the vision tests came back as normal (I thought they would). He's having a big three day assessment in June- and they'll look at his vision again then- but they've said that it's likely he has an "eye dominance" problem so he sees double when things are close. Not sure what they can do for that.....
I wouldn't worry about labels or getting remedial help - the children who really need extra help have to fight tooth and nail for every minute so they don't dish it out unless absoutely necessary (and usually when threatened with tribunals ) Believe me I know! (unfortunately) Let us know how youy get on.