{{explanation}}
3c start on opposite side (1,2,3x).  The “Active Couple (AC)” starts alternately from opposite ends.  When the “bottom” couple is active, “down” means towards the top of the set.  To start, 1c is the AC.  Six times through in all.

1–
: 1c lead down {3}, up {3} | cross and cast off (2c up) (2,1x,3x)
9–
: 1c+3c RHA ; 1M+2W & 1W+2M Turn LH (2,1x,3x)
17–
: 2c+1c L-Chain (2,1x,3x)
25–
: All A&R to P ; all turn RH (2,1x,3x)

{{explanation}}
Continue with 3c as AC.