The picture you have uploaded makes me think they might mean that you need to bind off at the beginning of two successive rows. So, the pattern may indeed be asking you to decrease three at the end of a purl row, but I think it would be unusual.
You might want to insert a lifeline in the row you have on your needles now if you haven't already. This makes it much easier to rip back if it doesn't look right to you. I love lifelines!
What I think the pattern may be asking you to do:
With the right side (public side) facing you bind off three stitches and then knit to the end. Turning the work you then bind off three stitches in purl and then purl to the end of the row.
Below is an example from this pattern I'm working on now.
Bind off 4 (4, 5, 5, 6, 6) sts beg next 2 rows."
Take a look at the sleeve schematic and then compare it to your own. I think this is the same thing your pattern is asking for.