Look at it like this.
Let's say you were doing a fight and nr1 on the meter was a mage, doing 10% of the damage.
You would be 6th having dealt 7% of the damage.
Add up the set bonus... tadaa!
That how much of a difference 3% can be. It might look small, but its huge since it's up basically all the time.
Look at some damage meters, usually there's about 5 or more players between a 3% difference.
Lets borrow a recent dmg meter screenshot:
Ok it's shit they didn't merge the pets with those DK's but whatever.
Take those Dk's, they both did 5.8%. Add up 3% would bring them up to 8.8% and look at where that would bring them!
Another example... take that hunter being down there all by himself... add up 3% to his damage and look where he would end
Yes I know those DK's already have the set bonus and blablabla, but it just to show you how much of a difference 3% on all damage can be.