Combining two sheets based on 3 columns

Question

I have two different sheets with 4 columns in each:

• USERNAME
• ARCHIVE
• ACTION
• TOTAL

Archive might be empty or a text.
Same username might have several rows.
Action can have one of two values (OP1 / OP2)

For example:

+----------+---------+--------+-------+
| USERNAME | ARCHIVE | ACTION | TOTAL |
+----------+---------+--------+-------+
| alex     |         | OP1    |   1   |
| blanca   |         | OP2    |   2   |
| blanca   | P315    | OP2    |   1   |
| carlos   | P711    | OP1    |   2   |
|    :     |   :     |  :     |   :   |
|    :     |   :     |  :     |   :   |
+----------+---------+--------+-------+

I want to merge two sheets based on first 3 columns:

+----------+---------+--------+-----------+-----------+
| USERNAME | ARCHIVE | ACTION | TOTAL SH1 | TOTAL SH2 |
+----------+---------+--------+-----------+-----------+

Norw that a combination (USERNAME,ARCHIVE,ACTION) might appear in one sheet but not in the other one.

Any simple way to do it?