3 ?? 

apparently there are three correct answes... 3,5 and 7 depending on how your brain works. I thought 5 and get the 3 answer but can't the 7 one

Yes, my first thought was 5 ....then I thought , no , that's too easy ....but for the life of me I can't work out 7  

The only way you can get 7 is if there are two separate lines of three ....and one duck between those two lines  

my brain got 7 

2 ducks in front of a duck makes 3 then 2 behind a duck makes 6 and one in the middle makes 7 QUACK!!!!  

Imagine 7 ducks numbered 1 to 7 and waddling right to left:
1234567

Duck "3" has 2 ducks in front of him; duck "5" has 2 ducks behind him; duck "4" is in the middle,

My immediate reaction on seeing this was that the answer was "3" (and I would still maintain that's the "expected" answer), However, I then realized that the puzzle quoted in the OP isn't using the normal wording, and - as is often the case with these sorts of puzzles - it's that which is causing the ambiguity.

The uusal wording is:

"When in line, there are two ducks ahead of a duck, two ducks behind a duck, and one duck in the middle."

It's the commas that are really important here, as they split the sentence into 3 statements, each of which has to be considered separately. As it's worded in the OP, "7" is an acceptable answer because the absence of commas allows you to consider the sentence in its entirety.

I said 5 initially but thought  it was a trick question  so thought again and said 3.

my first thought was none cos they were swans and it was a trick question. .

then I thought 5

 Very ingenious

Sam Loyd was a creator of popular puzzles over a hundred years ago. One of his most famous puzzles was called Get off the Earth. It was printed on two pieces of card with the inner card being circular which could be turned. In one position 13 characters could be seen, but in the other position 12.

that one is easy to figure out ..at the bottom in one position there is one character in each position around the globe but when turned there are two facing each other bottom left side, which makes the 13th character

The characters are drawn so that in the 13 position one character is entirely in the globe, then each character working in an anticlockwise direction is positioned so that more and more of the character is shown on the outside until the last character is shown entirely outside. In the 12 position, one character is almost all in the globe but the feet are outside making it slightly larger than when in the 13 position. The same applies to the other characters making them slightly larger than when in the 13 position.

My answer was five ducks. Two in front, one in the middle and two behind.

That's what my brain said too  

3.7