Three sheets to the wind.
An incorrectly 'sailed' boat lurching 'drunkenly' in a strong gale. or something.
The three sheets are the two ropes attached to the jib sail and the single rope attached to the main sail, if they are 'let fly' it can be very difficult to get them back on board, the vessel is completely uncontrollable with the sails out to one side and no easy means of hauling them back inboard (other than to lower the sails)