Not only has the Washington National Opera been presenting a world-class “Ring” production this month, but it’s featured no fewer than three excellent Brünnhildes, at a time when finding even one is a rarity. Catherine Foster was Brünnhilde for the first two cycles; Christine Goerke replaced her in one “Die Walküre” due to a leg injury; and now, for the third cycle, Nina Stemme has, as scheduled, stepped in, appearing in “Walküre” on Wednesday night.