Files with the .k3g extension are most commonly associated with the mobile GOM player. The GOM player is a media player that is popular with many P2P file-sharing software users. A mobile version of the software was created for watching movies and videos on cell phones and other mobile devices. The media files used by the software contain the .k3g file extension.
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If you have additional information about the K3G file format or software that uses files with the K3G suffix, please do get in touch - we would love hearing from you.