Probably just a slight adjustment will fix it and leave everythign in line. Sometimes if it is close, only one or two cogs are effected by a cable tension that is slightly off. Is it skipping that third cog going up, or going down, or both? If the slight adjustment of cable tension doesn't do it, also check to see that there is no damage to that cog (bent teeth etc) and that your derailleur hanger is alligned properly. (It should be perpendicular to the ground)