Season 3 should start around the same time as patch 2.3 is released, but please keep in mind that season 3 may not start on the same date the patch is launched.
We have said that were planning to give a two weeks notice on the start of season 3, which still holds true. If there are any changes on this plan, we will let you know. (Vaneras)
The interesting thing here is that they say season three will be roughly simultaneous with patch 2.3, and they will give two weeks notice before the start of season three. Does this mean we'll have two weeks notice before 2.3 is released? One can only hope, although it is historically improbable for Blizzard to give a patch ETA that's more than a few days away.