Three particles move in a straight line with initial velocities $v_1$, $v_2$ and $v_3$ ($v_1<v_2<v_3$), each with constant retardation $a$ such that motion continues for more than one second before the velocity of each particle becomes zero. If $s_1$, $s_2$ and $s_3$ respectively are the distances travelled in the last one second before velocity becomes zero, then