subtract multiple cells if conditions are met

I am trying to figure out how to make it so if column A on sheet 1 matches column A on sheets 2 and 3 it subtracts the number in column G on sheets 2 and 3 from Column B on sheet 1. I am not very well versed in excel but what i came up with is

Formula:

=B2-VLOOKUP(A2,Mercari!A3:H2000,7,FALSE)-VLOOKUP(A2,Ebay!A3:H2000,7,FALSE)

. This works but only finds the first value on each page and the data in column A appears more than once on each page. any help would be appreciated