Your camera was supported in LR4.
You have three options:
Subscribe to LR CC with Photoshop
Upgrade to LR6
Convert your CR2 files to DNG and continue using LR3
Or use Canon DPP to convert to tiff and import the tiff files.
As mentioned above you can use the free Adobe DNG converter to convert the files to DNG, which will allow you to import and edit them in Lr 3. Otherwise you'll need to upgrade to Lightroom 6.
DNG converter can be download from: