Inspired by this question:
Class skill +3, when you have 1 rank in that skill
OK, I have a fighter with a ranks in (for example) Acrobatics. Then I gain a level and add a Rogue. Now, if I started as a Rogue, or add ranks after the fighter multi-classes, I know he will get the +3 bonus, but if you have the ranks in the skill BEFORE multi-classing and then the skill becomes a class skill do you get the bonus or have to wait until you put more rank(s) into that skill?
Best Answer
For Skills when you multiclass all your skills immediately get the +3 class bonus provided you have at least one one rank in them.
Thus for your example:
Bob the fighter has 1 rank in acrobatics, and no stat bonus.
Bob gets +1 bonus.
Bob levels and takes 1 level in rogue. Bob now get +3 to acrobatics as acrobatics is a class skill for rogues, even though he bought this rank in acrobatics when he was a fighter. Where the rank came from is irrelevant, now Bob is a rogue it's a class skill for him.
Bob now has +4 bonus.