Is there things for 3 year old to do

Yes we have taken my son when he was 1.5 and just last week at age 5. There are things for all ages. They even have a tinker station for very young ages to build things right now.

Yes!

I do a membership because it pays for its self and you can go whether rain or sunshine unlike the zoo and other places. Wonderful place for kids of all ages.

I would suggest a membership. I don't think it's worth paying $30 or $40 for one day when a membership will pay for itself in 2-3 visits. It's a great place to go when the weather isn't great, and the planetarium is the best!