The two words that are repeated frequently in the third stanza are "happy" and "forever." Remember that the speaker of the poem is commenting on a scene that he is seeing painted on a Grecian urn, and thus he is talking about the emotions of the scene he is looking at and how those involved in the scene will experience those emotions and joys forever:
Ah, happy, happy boughs! that cannot shed
Your leaves, nor ever bid the Spring adieu;
And, happy melodist, unwearied,
Forever piping songs forever new;
More happy love! more happy, happy love!
Forever warm and still to be enjoyed.
The repetition of these two words could be said to have two purposes. Firstly, it could emphasise the joy that the speaker has and his enthusiasm for everlasting art, which of course the Grecian urn is a symbol of. Secondly, it could be seen as an ironic comment on the stasis and unfulfilled passion that the scene on the vase represents. Throughout the whole poem Keats seems to offer a certain ambiguity surrounding everlasting art, and this could be said to be captured though the use of repetition in this third stanza.