Cannont connect Ear Force X3 with my HDMI set up .

Is the problem that you're using a single HDMI cable to connect your Xbox to your TV? If so, this Turtle Beach article should help... ________________________________________
HDMI and Connecting Ear Force Xbox Headphones
The current Microsoft Xbox 360 series of gaming consoles allow the use of HDMI cables for simpler hookup to home theater systems and Hi-Definition televisions (HDTV).
The Xbox 360 Elite (the black one) comes with two additional cables, an HDMI cable and a separate AUDIO cable that provides a pair of RCA jacks (red and white) and an optical digital audio output (S/PDIF Out). With this cable package, you can easily connect any of our X-series headsets and an HDMI-connected television at the same time.
This Xbox 360 HDMI Cable package is available as an extra-cost option from Microsoft: http://www.xbox.com/en-US/hardware/x/xbox360hdmiavcable/
Here's a more in-depth explanation of the Xbox 360 HDMI Cable and how it works: http://support.microsoft.com/kb/937899?sd=xbox
[NOTE 1:] The standard Xbox models do not come equipped with this cable set. The HD cable they have obscures the HDMI connector, so that the two sockets cannot be used at the same time. If you only have this stock HD connector (component video, composite video, stereo analog audio and optical digital audio connectors), then you will not be able to use the HDMI connector at the same time.
In order to use both sockets at the same time (HDMI and A/V) you'll need to purchase the Xbox 360 HDMI Cable package described above.
[NOTE 2:] The current Xbox 360 Elite Holiday Edition (Fall 2009) does not come with the HDMI Cable set.
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Some companies make aftermarket HDMI cables for use with the Xbox 360, but these may not provide the dual RCA jacks that carry the 2- channel analog audio necessary for hookup of the anlog-only Ear Force X-series headsets. If this is what you have, you may need to find another way to connect your Ear Force X-series headset.
The TOSLINK digitally-connected X-series headsets can be always be connected using optical digital "Toslink" connections (and that's the best way to use them!).
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The oldest white-and-gray Xbox 360 models DO NOT have an HDMI port. With this particular (obsolete) model, you cannot use HDMI connections, but you can use any of our X-series headsets.
Current Xbox 360 models all have separate HDMI and A/V ports ("sockets") on their back panels.
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Here are some suggestions for alternative connection schemes:
XBOX 360 AUDIO PLAYS THROUGH TELEVISION SET:
For X4 and X41: Your best bet would be to connect the Xbox optical digital audio out to the X4 transmitter's optical digital input. Then connect the X4 optical digital output to the optical digital audio input on your HDTV or home theater amplifier.
[NOTE:] Some HDTV systems have an optical digital audio output that will not transmit Dolby Digital/DTS encoded digital audio, only stereo PCM audio will be passed. In this case, you must connect as above.
For X2, X3 and X31: You can use the Headphones jack or the Audio Out jacks on your TV to connect to the Audio In jack on the Transmitter.
For X1 and X11: You can use the Headphones jack or the Audio Out jacks on your TV (if available) to connect to the headset's analog Audio Input.

NOTE: Please make sure your TV has Audio Out jacks. Some TV's only have Audio In jacks.
Audio In jacks only accept incoming audio signal; they do not send audio signal out. If no audio signal is being sent to the headset, no sound will play from the headset.
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You may need to purchase a suitable adapter cable in order to connect the TV audio output(s) to the Ear Force X-series headset's transmitter or Audio Input plug.
- Connect the HDMI cable so that the Xbox 360 plays its audio through the TV.
- For X1, adjust the Dashboard > Settings > System > Voice in the Xbox 360 so that the talkback and game sounds are played together (Play Through Speakers). - For X3, X31, X4 and X41, you should set this to Play Through Headset.
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XBOX 360 AUDIO PLAYS THROUGH HOME THEATER RECEIVER (except X4 and X41): This is exactly the same as the above, except that you will use the receiver's Headphones output jack to connect to the Audio In on the Ear Force Xbox-compatible headset's Transmitter (X2, X3, X31) or the green Audio Input plug (X1).
- You may need to obtain a shielded audio cable with 1/4" stereo phone plug to 1/8" stereo mini-phone plug ends. The 1/4" phone plug goes to the receiver's Headphones jack, while the 1/8" mini-phone plug goes to the Audio In on the Ear Force Transmit- ter (X2, X3, X31) or Audio Input plug (X1).
- Adjust the Dashboard > Personal Settings in the Xbox 360 so that the talkback and game sounds are played together (Play Through Both). For X3 and X31, you may also set this to Play Through Headset.
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