"Three bears are in one house."
Translation:곰 세 마리가 한 집에 있어요.
I think your confused with the concept 하나, 둘, 셋, 넷, 다섯, 여섯, 일곱, ... and the concept 한, 두, 세, 네, 다섯, 여섯, 일곱, ... The former comes AFTER the noun, and the latter comes BEFORE the noun. ex) 집 하나(a/ one house), 학생 둘 (two students), 학생 셋 (three students) 한 집(one house), 두 집(two house), 세 학생(three students)
하나 is a numeral(수사) so it comes after the noun, and it CAN be used alone. Whereas 한 is a determiner(관형사*) so it comes before the noun, and it CANNOT be used alone. ex) 하나보다는 둘이 좋다 (Two is better than one) 오늘, 두 사람과 만난다. (Today, I meet two people)
*In Korean, there is a determiner called 관형사. 관형사 only serves to express a noun. There are 3 groups of 관형사s;
성상 관형사(a determiner which describes the noun) ex) 새 집 (new house)
지시 관형사(a determiner which points out the noun ≒ demonstrative) ex) 이 집 (this house), 저 집 (that house)
수 관형사 (a determiner which shows the number or order of the noun)
ex) 한 집 (a/one house), 두 집 (two houses), 세 집(three houses), ... It's similar with 'un', 'deux', 'trois', 'quatre' in French but the difference is that it only describes a noun. You MUST have a noun to use it (it cannot be used alone, it's not an adjective nor a verb nor an adverb). 한 is in this group.