pleTriwapt Sgpin (Triplet Swapping)
J, 13 bytes

[:;_2|.\_3&lt;\]


Try it online!

Example input: 'flybuys'


_3&lt;\] Box every group of 3

┌───┬───┬─┐
│fly│buy│s│
└───┴───┴─┘

_2|. Split that into groups of 2, and reverse each one:

┌───┬───┐
│buy│fly│
├───┼───┤
│s  │   │
└───┴───┘

[:; Raze the result:

buyflys