Legoland is not all about legos. Inspired by, and made out of, but you don't have to be a
lego fanatic to enjoy it. I was just there this afternoon (with a 6yo). About the only 'land' he wouldn't do is techno-land (that's what I call it, not what LL calls it!) where the Bionicle-inspired stuff is.
I think that a 3yo could have a lot of fun. But LL is pricey for a day's admission. And SeaLife is a separate admission, so don't get unpleasantly surprised!
There's a scaled down Buzz Lightyear ride, a teacups-type ride, a Dumbo-type ride, a Goofy's Barnstormer, lots of coasters for those of all ages (down to 36" for several, then most of the rest at 42), a Beetle Bounce (there are no words...), Duplo playground, Mini-Land (where all the scale replicas are), a silly Knights Joust ride, a water play place, the Volvo driving school (3 levels, for all ages), some sort of a jungle cruise, a log ride. Fairy Tale Brook is lame (IMHO), though DD always wants to go on it. I'm sure I've missed some. Oh! The BYO-bucket sand pit!
And let me go on record that our burger lunch there was better and cheaper than what you get at WDW! Black Angus burgers, chips, and a larger beverage.
At 3, your son can enjoy many of the attractions but not all. At almost 7, my daughter can do everything in the park (and she's not a coaster fanatic) and still enjoys it.
Deb