If the adults want to go, then you can find things for a 3 year old to do.
If you're trying to gear the park days around the 3 year old, IMO that's a mistake.
And it's nice to have the child be earthside and grow a bit so everyone knows what sort of 3 year old it is, before deciding on themepark trips.
I also, with a background as a daughter in law with a difficult relationship with her mother in law (first time I met her she tried to make me cry the whole evening...she's a joy), hope you're not overwhelming your adult child and his/her spouse with these expectations for a trip with a grandchild who hasn't even been born yet. With my own personal background, I'm twitching a little bit thinking about it! (and I'm not saying you're like my MIL, I'm just saying that with my background that is what is happening in my head)
It's lovely that it sounds like you want to take them, that's nice (my MIL won't even pay the $40 a month for the blood pressure medication that has literally gotten her out of the hospital twice and expects us to pay for it if we want her to take it), and definitely go ahead and start saving up...but 3 years from April-ish is a long long time and many things can happen. (in fact, go ahead and decide that baby will arrive in May, so there's no pressure being put on its mom...be a rockstar in that way!)