How to mix PS3- and PC-Sound for one headset? [closed]

This is going to depend heavily on a variety of factors such as the TV you have already and the features of your PC soundcard;

Does your PC have a suitable audio input to run your PS3 through the PC?

If your PC has something more than a standard on board sound card then you might have dedicated digital inputs for audio, if not then you should at the very least have a line in connection which should allow you to run the audio from your TV into your PC.

If yes - you can use the Windows mixer or mixer that accompanies your soundcard in your PC, and plug your headset into your PC and do the mixing there.

Does your TV have a suitable audio input to run your PC through the TV with your PS3?

Many TVs will allow you to play multiple audio inputs at once, for example HDMI and the audio inputs (red and white RCA) from composite video. If yours does - you can plug your headset into your TV, your PC into your TV with a cable that has a standard jack on one end and red/white RCA on the other and adjust the volume levels of TeamSpeak using your PC to get the balance right.

Failing that...

You're going to need some kind of hardware solution, whether this is replacing the soundcard in your PC to allow you to relay the audio from your PS3 through the PC or getting something like an external mixer or amplifier of some kind which will require a variety of cables to plug both your PS3 and PC into it.