We find that two working together will do more than two working individually, however if you add a third person to the equation the amount of work will not increase in line.
Basically, if doing trad you'll have one doing tops one doing bottoms, same with WFP, what does the third person do, they are forever asking "where next?" "have you done that one?".
You will make more than two working, but probably only 20% increase.