It means BO 3 sts over the next 2 rows, 4 times, so it involves 8 rows. You can't BO at the end of a row, so you BO at the beg of the next row which would be on both sides/edges of the sleeve.
RS  BO 3, finish the row
WS  BO 3, finish the row
RS  BO 3, finish the row
Ws  BO 3, finish the row
RS  BO 3, finish the row
WS  BO 3, finish the row
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sue knitting heretic
