If I see three sites where I would like to advertise and each of the three sites has AdSense, is it possible to open an AdWords account for the sole purpose of targeting those three sites or does an advertiser need to have some sort of seniority or length of time with AdWords to do this?
netmeg is quite correct that you can open such an account - however there is an important caveat I'd like to add:
Not every site which has chosen to show AdWords ads will allow their site to be specifically targeted.
Put another way, while a site owner may indeed show AdWords ads on their site, they may have elected to not make their site available for site targeting.
AWA