I have two files:

file1.txt
l l 0
s s 0
a a 0
a b 0
a c 0
a d 0
a e 0


file2.txt
a a 1
a b 2
a c 3
a d 4

Both this two files have 3 colums.
The file1 will have blank records and the file 2 will have values but just for some combinations.

I need to created an file3.txt that will have a blank record in case the file2 don’t have a value for that combination.

File3.txt
l l 0
s s 0
a a 1
a b 2
a c 3
a d 4
a e 4


Can anyone help me in developing this in shell script?
Thanks