The third season of "I Want to Sing with You" is an interactive music program launched by Hunan Satellite TV. It is broadcast on Hunan Satellite TV at 10pm every Friday. As for why the 12th issue is unavailable, there may be the following reasons:

1. Program copyright issues: It may be that there is a problem with the program copyright, which prevents the 12th issue from being broadcast on the TV station.

2. Host or guest issues: If there is some negative news or controversy about the host or guest, it may affect the broadcast of the program.

3. Program content issues: It is possible that the 12th issue was banned from broadcast by the censorship department because it involved some sensitive topics or content.

No matter what the reason is, it may be impossible to watch the 12th issue. If you are very interested in this, you can try to search for related content on some video websites or APPs. There may be clips or related videos uploaded by other users.