三人いるのに = even though there are three people 食料が二人分しかない = there’s only enough food for two people (しか followed by a negative means “nothing except x” or “only x”) みたいな時に = at a time like that
武器をとって戦うしかなくなる = become so that all you can do is take up arms and fight んだ = explanatory tone きっと = surely
For example, in a situation when there’s three people, but only food for two, there’s no choice but to take up arms and fight, surely.
This sets up the headbutt scene
She’s pretty hungry 
I wouldn’t say she’s serious. But it leads to her next question.