"There’s a pretty sweet fight scene in episode four. It ends in the death of a new significant character and further shakes up the political landscape."
Source: http://gazette.com/the-100-season-3-preview-spoiler-free/article/1568003
New characters we know of so far:
1. Queen Nia
2. Roan
3. Ontari
4. Titus
5. Niylah
6. Pike
Could always be one we don't know of yet, because IMO all of those characters seem too important to kill off right away, don't they?
We see Lexa kicking some dude off of a cliff in the trailer, maybe the fight's between them? He looks like a Grounder, so maybe he was sided with the IN and opposed Lexa's rule.