Yes you can if site owner agree with you. But be careful that third party site should not violate ADSENSE TOS.
Yea you can do it, there is no any restrictions from Google to paste code on other members site. Just be careful about rules and regulations.
You can do that if the site owner agree with you but you have also to take the risk that your account might be ban if they place your ads in a site that don't follow google adsense TOS.