3 is not an MVNO. They may have recently set up a network sharing agreement with T-Mobile, but that doesn't make them a virtual network. The basic idea is that all existing T-Mobile 3G towers now transmit both T-Mobile and 3 identifiers, and same goes for the existing 3 ones.
However, their network is a bit patchy, but their data plans aren't too bad - but watch their out-of-bundle rates of 10p/MB on contract or £1/MB on PAYG.
Compare to Orange's "mobile broadband" offering where their bundles might not be quite as generous but out-of-bundle is 1.5p/MB.