matching and extracting data

Dear all,
I have querry on how to extract the data by matching between two  
data set where one has the same elements multiple times?

For example, I have two matrix X and Y.

X

   [,1]    [,2]       [,3]
1  A     5          P
2  B      6         P
3  C     7         P
4  D     5         Q
5  E      6         Q
6  F      7         Q
7  G     5          R
8  H     6          R
9  I       7          S
10 J      5         S
11 K    6          T
12 L     7          T

and

Y       [,1]

1          P
2          Q
3          R
4          S

Now, I want to select and extract all the data of P, Q, R and S  
elements of column 3 of X matrix by matching with column 1 of Y  
matrix like below:

[,1]   [,2]       [,3]

1  A     5          P
2  B      6         P
3  C     7         P
4  D     5         Q
5  E      6         Q
6  F      7         Q
7  G     5          R
8  H     6          R
9  I       7          S
10 J      5         S