I am using Lightroom V 3.6.
G15 came out after Adobe quit maintaining Lr3.
Yes, Adobe could keep retrofitting old software with new camera support, but they don't.
Note: if they did, the money to do so would be coming from users who pay to upgrade, and only benefit users who haven't paid to upgrade.
Raw files from newer cameras will not be read!
Use the DNG Converter to convert your RAW files to DNG. The DNG files can be opened by your version of Lightroom. To my knowledge, there is no (essential) information lost by the conversion process.If you are unsure about if you ever have a need for the original RAW files, you may incorporate those into the DNG file (makes bigger files...) or archive the RAW files.