The increases are each side of the middle stitch.
The bind offs are at the beginning of a row so they are at the very edges of the knit piece.
Since you can really only bind off at the beginning of a row, bind off 1 stitch at the beginning of the knit row, work to the end of the row, turn and bind off at the beginning of the purl row. That way, the two edges will decrease and the shaping will be the same on both edges.
Call the first bind off row, 0 inches, then you would bind off when your knitted fabric measure 3”, 6”, and 9”.