I am planning to fly to SG during holiday in April. Beside of Universal, is there any activity place that suit for 3 years old girl?

I would suggest Adventure Cove Waterpark. It's located in Sentosa Island, so it's easy for you to actually go explore other attractions over there. The place cater to all different ages, so the whole family should be able to enjoy it. Opens at 10am - 6pm daily and will cost around SGD $26 - $36 per person. Enjoy your time in Singapore! :)