Three pebbles forming a line on a 3x3 grid

Three pebbles are thrown simultaneously onto a $3\times 3$ tic-tac-toe grid. Each pebble independently lands in one of the nine cells, every cell being equally likely, and several pebbles may share a cell. Find the probability that the three pebbles occupy three cells forming a winning line (a full row, a full column or one of the two diagonals).

$16/243$. There are $9^3=729$ equally likely ordered outcomes. A winning line needs three distinct cells forming one of the $8$ lines, and the pebbles can fill a given line in $3!=6$ orders, giving $48$ favourable outcomes; $48/729=16/243$.