Currently, it isn't. When using one of the software rasterizer backends (the old, deprecated swrast, or the more modern, gallium-based softpipe or llvmpipe drivers), only GL 2.1 will be advertised. The issue is that mesa's software rasterizers do not yet support multisampling, which is a requirement of GL 3.x. There might be also some other minor features missing which are required for GL 3.x.
However, you can still use most of the GL 3.2 features via the extension mechanism, without having a 3.2 context. This also means that you won't be able to get a core profile context, but this shouldn't be a problem either - nothing forces you to actually use the deprecated functionality.