The first D is a Dinosaur egg - photographed at the Oxford University Museum of Natural History.
The second D is a repost of Down House, which is a museum dedicated to the life and work of Charles Darwin who lived there.
The third D is the Dee itself (clever, eh?): the river which forms part of the border between England and Wales. This is the estuary, looking south from West Kirby towards Flint in North Wales.