I have twin boys who just turned 4. One of my twins was always more advanced developmentally. Rolled over earlier, crawled earlier, walked earlier etc... That twin (Twin A) also was potty trained in a day when he was 3 (I waited until they turned 3 b/c I wanted to train them in the time between camp and school). Twin B, however, didn't potty train at that time. It took him another 6 months.
On the other hand, Twin B knows ALL his letters, can spell words and is starting to sound out very basic words (cat, bat, mom, dad). Twin A (the one who developed faster) basically did not know his letters at all (until probably about a month ago). When I'd show in the letter "A" and ask what it was, he'd say "it's a triangle!!"
I asked his pre-k teacher whether it was a problem that he didn't know the letters (after all, he's in school full day sitting with the alphabet all around him!) and she said absolutely not. He was on track and that, he just has to recognize his name. Indeed, they just want them to be able to write their names when they go to kindergarten. I, too, was nervous b/c Twin B is doing all these things with letters (as did my older daughter). They reassured me that Twin B and my older daughter were somewhat exceptional in this regard, and that Twin A is just normal and right on track...
But I see a huge difference in his ability in the last few months. Now Twin A can write his name, he knows letter sounds etc... So don't worry (yet) about your 3 year olds. Twin B may go from not saying the alphabet at all (which is what my Twin A was like) to knowing to spell his name really soon.
If you have any concerns, though, ask his teacher or pediatrician....