Lois just got relieved that her ex (Oliver) is out of the woods.
Clark: "Kitten adoption fair"? You gave up your article on Oliver for this?
Doesn't sound like you, Lois. Is it possible you're more shaken up about last night
than you realize?
Lois: You think? I was practically a three-shot martini.

She says even more shaken up?
Then Lois talks to Clark about how hard it was for her leaving her boyfriend
and that she had done it for a reason. And suddenly she says:
That one's on the house, Smallville(that's Clark). You should know,
I'm allergic to cats. (And gives Clark the 'kitten' article) Get to work. (Clark smiles)

What does she mean by that? Enough with the free advices or
she joked with the article, meaning she's not actually writing it?
three-shot martini... Yes, I think she's referring to how a martini is shaken when it's made. So she's saying that she was very scared.

on the house: free. I guess that Clark would be pleased to finish the kitten article and get credit for it.
Thanks, Vorpar!