3D automatically moves the focus point around in the viewfinder. If you are photographing a bird flying in an empty sky, it will work well. Otherwise, you run the risk of the focus point grabbing a tree branch. If you have any moving water such as a flowing stream, give it a try. That focus point will race all over the place.
Dynamic allow you to choose the number of focus points you want to include and will pick one to focus on what it thinks is your subject - providing your subject is beneath them. For example, if you choose Dynamic 25, the body will only use those 25 points to focus rather than all of them.
Have you watched Steve Perry's video which explains and shows differences? It might give you a better idea how they work.