The Song of the (three) Thames-daughters begins here. From line 292 to 306 inclusive they speak in turn. V. Götterdämmerung, III, i: the Rhine-daughters.
Eliot is referring us to Richard Wagner's opera Götterdämmerung, (Twilight of the Gods), part of the four opera cycle Der Ring des Nibelungen (The Ring of the Nibelung). Eliot wants us to think of Richard Wagner's drei Rheintöchter (three Rhine daughters) as we come across his own Thames-daughters.
The Ring of the Nibelung consists of the operas Das Rheingold, Die Walkure, Siegfried, and Götterdämmerung. The Ring cycle starts with Das Rheingold where three nymphs in the Rhine River have gold taken from them. The thief may, and does, forge the gold into a magical ring that gives power but only if the maker forswears love forever. This ring eventually comes into the possession of the great warrior Siegfried. In Götterdämmerung the nymphs attempt to persuade Siegfried to give them their gold back. He does not and is later killed for the ring by the son of the original thief. The Rhine-daughters get their gold back as the rising Rhine floods and extinquishes Siegfried's funeral pyre.
There is more about Der Ring des Nibelungen on the allusion page to line 277.
The songs of the three Thames-daughters are on lines 292 and 296 and 300.
Note that there are two references to (Tristan und Isolde), another Wagnerian opera, in The Waste Land at lines 31-34
and line 42
31) Frisch weht der Wind
32) Der Heimat zu.
33) Mein Irisch Kind,
34) Wo weilest du?
42) Oed' und leer das Meer.