Damage % stacks additively, if you start at 0%, and get 50%, you do 50% more damage. Then if you get another 50% you do 100% more damage than when you started, but compared to having a 50% bonus, it's only a 33% increase.

but with items like that sausage and those how does it work. he 3x1 and 25% fire chance. 4 of em does it become 12x4 100% ?

Damage % only affects the damage, so for sausage, the first number is the number of ticks that will happen as it's a DOT effect. So that number stays the same. The chance also stays the same since that has nothing to do with damage.

So for that example, getting 100% more damage as a stat would make it 3x2 with 25% fire chance.

so whats the point in getting 4 of them?

see this is why i want a mod to help wth items better

The other comment was talking about how damage % interacts with Sausage. If you stack them you only increase the chance to burn the enemies.

There's a mod called Improved Tooltips that shows more information but I haven't used it myself. I find most things self explanatory, and because you can see the numbers on the items change when you increase your stats, you can notice stat increases.

It's kinda nice to look at your weapons, buy an item and see how much your damage increased.