Congrats on your little ones development. MOE SG suggests to only have your child tested for giftedness after the age of 5. Take your time and enjoy preschool days :) By the way out of 100k population, usually about 3 are considered to be exceptionally gifted.
Try to check at Mensa Singapore : https://www.mensa.org.sg
Check with Thomson Pediatric Centre.
You could try Mensa Singapore