df1
col1       col2      col3      col4
 1           2        A         S
 3           4        A         P
 5           6        B         R
 7           8        B         B

df2
col5      col6         col3
 9         10           A
 11        12           R

df3
col1    col2    col3    col5   col6
 1       2       A       9      10
 3       4       A       9      10
 5       6       R       11     12