For a 3:45 flight, you will be picked up around 12:45....15 mins either way. You need to be back at your resort, all ready to board that bus no less than 15 mins prior to the time you are given. So, figuring on a 12:45 time, you have to be at the bus stop by 12:30. Which means getting back to the resort by noon so that you can get your carryon bags, use the bathroom, etc. Then, depending on the park, and the resort you are at, you could need up to an hour to get from the park you choose back to your resort.
Here's my recommendation...if you are planning on going to a park that is close to your resort (Epcot or DHS when staying at an Epcot resort, MK if at a MK resort, or AK if at AKL), then sure. But leave 30 mins to get back to your resort. If a park has EMHs that morning, head there.
If I have a flight that departs after 4pm, which gives me a p/u time of no later than 1pm, I like to head to a nearby park...that isn't dependent on buses....that has EMHs. That gives me almost 4 hrs in the park.
If I have less than 3 hrs in a park, I forget about it, and head somewhere for a nice last day breakfast. If at an Epcot resort, I head to Goofy's Beach Blast Breakfast. If at AKL/Kidani, I head to Boma. At a MK resort, over to 1900 Park Fare or 'Ohana. I have also been known to head to a park just for breakfast....Tusker House, Akershus, Crystal Palace. But I get a very early seating.