Ah, I see! Yeah, that's like, super old-school PHB wording... technically true, you can do that... but you can also just spend ranks (it doesn't, after all, say that you must do that). On the very same page: "3. If you have not maxed out a skill, you can spend extra skill points on it and increase its rank further." ... basically, you spend ranks on a 1:1 basis, or 2:1 if it's cross-class, that's it, just the same as at L1. The 3+level thing is just to max it out, I guess because they assumed you would be doing that (a lot of old PHB stuff is written that way). Pretty sure though that everyone has more or less just been ignoring that line for the past 20 years since it's not super helpful.
There's no reason you can't just put, like, 3 ranks into a skill, and call it a day (even at L20).