My DS17 was obviously gifted as a preschooler, he read at 3 but I didn't have him tested till he was 15. My DH is profoundly gifted so it didn't come as a surprise. My older DD21 and is NOT gifted she tests above average but with notable LD's and has struggled in school. Thankfully they are 4 1/2 years apart so the older one didn't compare herself too much to him.
As to your comment that DS3 doesn't seem gifted because he's not reading like his sister. I've known many very gifted kids who don't read till they were 7 or so and never do anything "showy". You will see of this board many a parent say they didn't have a clue till they started to be very bored in early elementary.
DS when 3 also did that thing where skills came out of nowhere. One day out of curiosity I asked him if he could write his name having never seen him write a letter in his life. I figured we could work on the first 3 letters. He proceeded to write his name & then announced to me "now I'll write the alphabet". And except for a few upside down letters it very very legible.
