I have an idea that the elves would have some archers in their faction. Just thought of that from watching Lord of the Rings. :P

Also I am wondering what the elves faction would have to compete with: 
The undead having a military similar to usa modern military and humans with their created cyborg super soldiers. 

Maybe for a plot there could be a God from the heavens, who has appeared to each faction separately and explained to each faction why the other two are evil. 

The God could also have explained about a reward that would be everlasting if they were the only faction left living on the planet. 

Then later on after trying to wipe each other out the factions could have seconds thoughts, about what the god told them after having suffered through war. 

Then the God might become angry with the factions second thoughts about what he/she told them and might decide to try and punish/wipe them out personally.