You "seem". All are correct. Which to use depends on the context. The context you provided is not sufficient. I would need to hear the rest of your conversation with your friend to decide.

They are professional enough， I have nothing to say.

They are all unclear and unnatural. If the person is in Korea, you aren’t observing them work in the US. So the verb ‘to seem’ by itself isn’t correct. It doesn’t make sense to talk about ‘country’ It seems like you work hard in the US. It seems like you work hard there. It seems to me that you work hard in the US. (These versions of ‘seem’ don’t require a direct observation) You seem to be working hard here. You seem to be working hard since you’ve arrived here. (‘Seem’ could be ok here because you are observing their current behavior) Best is to ask a question: Do you work harder here or in the US?

I'd say 1 and 3 make sense for what you want to convey. Number 2 has a pronoun, so it wouldn't be suitable unless you've referenced the country in a previous sentence. By the way, the second person does not need an -s, so it should be "you seem" instead. Hope it helps!