They would go over a 3' fence with no problems so clipping one wing would be best to keep them home and even doing that they may get up onto a 3' fence mine can. But I see for one the ramp is too steep and not wide enough Ducks have a wider gate than chickens and don't do ramps that steep very well. But 2 ducks inside your enclosure would be fine if you plan on eventually to let them out to roam around the garden.The problem is we live in terraced houses and the gardens are seperated by 3ft high fences, and I'm sure the neihbours wont want them jumping into their garden... I'm starting to think this is a mistake. I'd love them but I really don't want to compromise their care in anyway.
Welcome to BYC!