As of right now I don't believe any 3rd parties have firmware which will work properly with the v3s. For now, it would be safest to stay with the stock Linksys firmware. However, there was someone on the hyperwrt.org forum who said that they heard that the stock firmware wasn't even stable with the v3s. So you folks might be out of luck until someone figures out how to get this working properly.
As for Sveasoft compatibility, one of the devs said that they had acquired a few V3s to experiment with. As far as a firmware release which supports these units, I have no idea what their timeline is. There is a lot of discussion on this topic in the Prereleases forum, take a look.