Comparing and referencing to data

Hi All,

I have a sheet in which one column needs to be populated based on data of two columns. The scenario is :

Sheet1
column1(M) column2(P) column3(B)
M1 P1
M2 P1
M1 P2

Sheet2
column1(M) column2(P) column3(B)
M1 P1 B1
M2 P1 B2
M1 P2 B3

Now, based on info given in sheet 2, coumn3 needs to be populated and if no match is found user is asked to enter data in sheet2 first and then proceed in sheet1.

Please help me to do this.