I have 3 companies, and want to compare how many accounts are customers between these companies. 

the picklist is Customer, Prospect, Competitor

I want to do this with a row level formula for example

If company 1 = TRUE, and company 2 = TRUE it is Company 1 and 2 
if comapny 1 = TRUE and company 3 = TRUE it is Comapny 1 and 3
if comapny 2 = TRUE and company 3 = TRUE it is Company 2 and 3