Column of probabilities

H all,

I have a 6 x 3 matrix. The last column is simply the sum of of the  
first two
rows.

x1  x2   x3
1     2    3
2     1    3
1     0    1
2     2    4
2     1    3
2     1    3
0     0    0

I want to create a column of probabilities p, such that for each  
row, I want
to find the probability of x1=k and x3 = n divided by the  
probability that
x3 = n.
So I should end up with a column of probabilities:
1/4, 3/4, 1/7, 1/7, 3/4, 3/4, 1/7