"There are three people in the house."
Translation:Estas tri homoj en la domo.
It ultimately means the same thing, but it's a different construction. The original sentence implies "There are three people, and they are in the house", while your sentence means exactly what it says.
You may want to report it, although it'd be up to the DL staff to figure whether that can be considered close enough to the original intent. Sometimes it's important to get the translations word for word, and sometimes it's better just to maintain the meaning behind the sentence while changing the words.
"in the house" is a prepositional phrase. Prepositional phrases require a preposition (in) which is usually a word of relative position or motion (like in, around, through, over, to, with, without, by etc), and an object (house) preceded by its adjectives (the). In some languages these get special treatment, but not in Esperanto. The object simply takes its normal form.
The only time you need the accusative -n is when you have a direct object such as "I have a house". In this case, the object (house) is being described not by a word of position or motion, but rather by the subject and verb themselves. Unless that verb is a form of "estas", the object in this case will always get the accusative -n.
Here are some examples:
I am in the house - Mi estas en la domo (Domo is described by the preposition en rather than the verb (estas), so it is not accusative.)
I am a man - Mi estas viro (While viro is directly associated with the verb, it is a form of estas, so there is no accusative here.)
I have the house in the city - Mi havas la domon en la urbo (The house is directly associated with the verb, because it is being "had". But the city is only related to the preposition "in". I'm not having the city, so only the house is accusative, making it domon)