You do not indicate how many are in your party, but it may be cheaper to rent a car for those three trips. I rented a mini van for our trip to Sea World and the cost from Dollar was $45.00 with tax. If you are making that many side trips it would be worth it to rent for a week. The cost of using a town car service for those three days would be about $180.00 plus tips
Or if you are a smaller group you can go with Mears, the cost to each of the parks is about 12.00 per adult.
If it were me, I'd rent a car. You can rent from the car care center, the Holiday Inn in DTD has a Dollar Rental, I think that there is rental cars available at the Dolphin