Ideas

I will Present each faction in a certain way

Mutants : I will Portray the mutant faction not as clean cut as the other 2 factions, not fonts but more scribbles and slashes on the posters for the words as they do not have technology to write correct letters or how we would interpret them, and also anything that isn't a mutant will be their enemy

Brotherhood Of Steel : These will be portrayed as a more patriotic faction, they are more with trying to find peace and ending all the killings and murders but have the force necessary to counter any attack on them.

The Enclave : These will be portrayed as a race that see themselves as a more highly than everything else and generally consider most people outside the enclave to be mutants that are only fit for extermination rather than trying to help, and this will be portrayed in my posters.  
Here is a video of the Enclave to show you what they look like http://www.youtube.com/watch?v=y-_Jk9yorLg