Lightroom 4 was out almost a year ago. Lightroom 3 is not going to be updated.
However, here are two ways to use your 6D with Lightroom 3.Method A:
1. Install the DPP disk that came with your camera. Setup a separate folder for your 6D images, and use DPP to import them to that folder.
2. Use the Batch Process Button in DPP to export the images to a folder of your choice. The images will be converted to tiff which will work fine in LR 3. (The CR2 file is basically a compressed tiff image with Canon metadata added)
3. If you set LR 3 as the external editor in DPP, you can also invoke LR3 from within DPP to edit a image. Method B:
1. Download Adobe DNG. http://www.adobe.com/support/downloads/detail.jsp?ftpID=5519
2. Create a separate folder and place your 6D images in it.
3. Run Adobe DNG on that folder, it will convert the CR2 files to DNG.
4. Edit the DNG files in LR 3.(Your CR2 files will not be changed or deleted)
Either way will work fine. You will never need to upgrade LR3 if you wish to keep it.