I don't understand why in the film Renesmee has ten years if in the book is described as a little girl who was only three years old!!!

It would be very difficult to get a three year old to recite all of Renesmee's lines and walk around with mature poise as Renesmee is supposed to.

This isn't the first time the "age" of a character was changed from the book.
In the books, Aro's perpetual age is somewhere in his 20s, Caius is in his 40s, Marcus is 19, and Jane & Alec are 12.
It happens sometimes.

I think because there wouldn't be so much age difference between Jacob and Nessie.
Or it was easier to make a movie scenes with 12-year-old than three-year-old child.

I think Renesmee(Mackenzie Foy) is used later in the film to look like a really tall 5-7 year old, even though she might be 3. In the trailer, they used a baby too, so she's not always gonna be tall.

Most child actors are older than the characters they portray. That way, they can work longer hours than if they were the same age, they're more mature, they could have more acting experience, and they're less likely to throw a tantrum fit on set.