Ok, I would say the dog house would be big enough for 2 maybe 3 but you will want a much larger area for their yard or it will turn into a mucky mess very quickly.
As for ducks being sweet or aggressive it does depend on how much they interact with you. But breed, gender and individual personality also factor in. Some ducks and drakes are extermely friendly some are terrors. I have never been one to coddle my birds, I raise them for meat and eggs not pets but I have never had an aggressive drake and they all come up to me for food/treats. They do try to get away if I have to pick them up for something but they will come close enough that I can grab them quickly if need be.
You mention the destructiveness of chickens scratching. Give ducks a puddle and your grass will turn into a mud pit that nothing will grow in.
Since you have a small area available for them, if you decide to get ducklings, I would suggest getting sexed ducklings so you know what you are getting. If you decide on 3, then you definitely don't want 2 males.
There are a number of things that you need to look into before asking if you can get them. Who will care for them? Ducklings are similar to chicks but WAY more messy and smelly. They have different dietary requirements than chicks but do need much less elaborate housing. I also suggest getting a book, check your local library, so that you can read up on what you are getting into. Storey's Guide to Ducks is great and is my go to for info.
Think about what you want them for. Eggs, meat, ducklings, lawn ornaments. These things will narrow down the breed. Also, most ducks will be loud. Again, breed and individuals will differ but some breeds are more chatty than others.
If I had to choose, I'd have ducks instead of chickens. Luckily, I have room for all.
