"There are three temples in this city."
Translation:Es gibt drei Tempel in dieser Stadt.
“Da sind …” is not accepted here. Is it wrong to combine “Da sind” & “dieser Stadt”?
As "there is" and "there are" are English idioms to say "exist(s)", I would prefer the German idiom with the same function - "es gibt" - over the literal translation "da sind". If I hear "there are three temples in this city", I don't imagine someone pointing to the temples, but rather a statement about the existence or the number of the temples. The location is given by "in this city" instead. If "there" would be the location, it could very well be "da sind" in German ("Look, there are three temples!" -> "Schau, da sind drei Tempel!").
I don't think that it is really wrong, and I don't know any rules against it but the solution of Duolingo definitely sounds more natural.
I think, Matthew, that sticking with "es gibt" is best, as this sentence is about the EXISTENCE of the temples, but I'd say you don't need the comma. "In dieser Stadt gibt es drei Tempels" is fine. "In dieser Stadt" is one concept: that of position. Thus your verb IS in the right position...........i.e. the second position.