How to connect different criteria among 3 sheets?

I have a workbook with 3 sheets that i'm hoping to tie together.
sheet 1 is overview, list of salespeople and their monthly sales, in total.
sheet 2 is parameters of each item sold: id number, seller, client, various costs etc
sheet 3 is huge amount of data specifying compensation amounts from the manufacturer. each item sold can have up to 20 different entries.

connection between 1 and 2 is salesperson's name
connection between 2 and 3 is unique id of each item

i need to get compensation amounts applicable to each salesperson in current month from 3 as a sum to 1


Do you, kind people, have any ideas?

I can't figure this out.