I was considering this question and thinking it should be possible to figure it out on paper mathematically without doing a bunch of knitting to solve it.
Yeah, right. So much for my math skills.
So then I thought the only way I could do it was to cast on 251 and follow the instructions.
So I cast on the 251 (that is a lot of stitches by the way
). Then I got intimidated by the number of stitches and was afraid that the only way to be certain I did it right was to get it right the first time or do it again the "other" way I might have done it after my first failure. Then
I wised up a bit and tried it Suzeeq's way over 25 stitches of my 251 and counted how many I ended up with: my result 6. Then I tinked those back and did the 25 the other way: result 10 left, so I guessed that must be the way to do it because the problem was we were getting left with too few.
Then to check my hypothesis I did it over the whole 251 and I have to admit my result was 100 instead of 101. (I'm not sure where the stitch was lost. I wondered if that first knit stitch should have had a mate like the rest of them, but the way it is written it wouldn't.)
Here is what I learned. You need to end up with 2 not 1 st between the cast-off 3s. Here is how I did it:
K1, then knit 2 for the beginning of the first cast-off three. Lift your first stitch over. K1, lift over. K1, lift over. Now begin the repeat: K1, K2 more then lift 1 over, k1, lift one over, k1, lift one over.
When you finish the cast-off 3 you have one on the RH needle. You end up knitting 3 more (for a total of 4 in a line) before you lift the first stitch over. You end up with 2 sts, the 3 cast off, 2 sts, the 3 cast off all across, except that first one that is all alone, and my far end didn't come out like it seems like it should have. It seems like it should end with a K1 leaving 2 together right at the end. I kind of think you need to start so that you have 2 together and end with 2 together like all the others, but 251 stitches is a lot of stitches to have to keep "trying ideas" on.
I hope this helps anyway.