The buttonhole will be where you bound off the 3 stitches. Finish the row after the bind off, and when you work back on the next row, cast on 3 stitches where you bound off on the previous row.
I'd suggest knitting cast-on or cable cast-on rather than backward loop, though. They produce a tighter edge.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry