decrease on ribbing? help...

[kashadilla]

I consider myself a beginning intermediate knitter..every once and a while when reading through a pattern before beginning I find something that I'm not exactly sure how to go about doing. in the sweater pattern I'm currently doing, the directions for the neck shaping go like this:

working in k3, p3 rib, inc 9 sts evenly across sts on each side

How do I do this and still maintain k3 p3 ribbing? I may just be imagining this, but won't an inc in the middle of a rib make that rib a k4 rather than k3? I guess I'd just like to get an idea about this before I dive in; I'm rather paranoid about making mistakes! thanks!