So at the end of Row 3, you will remove the traveling marker, at this point you will have 3 stitches left to work on this row. Row 4, you will slip 1, k2.
This is when you begin your next triangle. The instructions direct you to knit to within 16 stitches of the end of row. So you will place a marker here, turn, slip 1, knit 2, place another marker (this will be a traveling marker) knit to the end of the row and then cast on 16 stitches.
As a side note: The number that I used to stagger was divisible by three (Wingspan directions instruct 16 for sockweight yarn). There was confusion for me otherwise, don't remember exactly why I was thrown off, but the multiple of three worked great.
LunadelaOssa on ravelry.com