If it said,
Dec every 3rd row on the right edge you would dec every 3rd row on the left edge.
If it said
Dec every 3rd row at the end of the row you would dec every 3rd row at the beginning of the row.
You just put the shaping on the opposite edge from the first piece. Thin might include binding off on the WS because you can’t BO at the end of a row, only the beginning of one.