Flame body does not stack up when breeding meaning that it will only count as if one Pokemon has flame body not all three so it will just be half as many steps as it normally would take to hatch it so it is pointless to have the other two flame body Pokemon. Source: http://bulbapedia.bulbagarden.net/wiki/Flame_Body
Half the time it takes for 1 egg to hatch, as Flame Body doesn't stack in breeding.
Source: Experience
Flame Body doesn't stack, so half the time.
Source
