Marging 2 files by matching 3 columns / VBA

Hello,

I have 2 fies that I need to marge to 1 file, one a week.

The match between them are with 3 fields (columns). 
The files export from the system(Web app) each time.

In addition, after the marged, it should be create some new columns with some calculation.


I'm looking for a solution for that, I'd like to here how I can resolve it.


Thanks