I'm new to pathfinder and I was looking at my friend's character sheet and it could deal 1d8+3 damage. Where did the +3 come from?
There are a LOT of things that '+3' could be. One of the things about rules heavy systems is that primary stats (like damage) need to incorporate modifiers from a massive number of disparate sources in order to make all of the various rule subsystems non-trivial, in order to allow players to benefit from interacting with those rule subsystems. That means that the damage dealt by a weapon is going to be modified by everything from whether or not you're underwater to how much you hate goblins, in addition to more obvious things like what weapon you're using and how strong you are. Now, you mentioned that this particular formula was on a character sheet. Normally, when I make a character sheet, I put down as damage all the things I expect to normally be affecting my damage when I attack, and then the things that only affect it sometimes we remember to add in when the attack actually happens. Assuming that's what your friend is doing, the '+3' comes from their 'normal' bonuses and penalties to weapon damage. Below is a list of some of the more common ones: