According to HBO.com, Big Love Season 3 was supposed to have started on December 14 in OnDemand. It's not there. Actually, all Big Love episodes are no longer listed. Big Love isn't listed as a series in the HBO OnDemand.
What happened? Where is the Big Love OnDemand?
I'm not really sure what was going on, but I can assure you that Big Love is available on HBO On Demand. Try checking now. If it's not there send me a PM and we'll troubleshoot. It's possible that there might be an issue with On Demand in your area, but I don't see anything of that nature at this time.