This one is worded very strangely. I think you work 5 sts then decrease, work across and 7 sts before the end do the dec and work the last 5 sts. It sounds like they may be unbalanced decreases, more at the beginning of the row than at the end. The 3-1 sounds like a double decrease taking 3 sts down to 1 maybe ssk. But then the following decreases I'm not sure if you dec 2 sts on the next RS row, one at each end, or dec 2 sts at the beg and one at the end, or if you dec 2 at both ends twice then dec 1 st once.
I think this is one where you need to contact the designer and ask to have it spelled out. I don't understand it myself.