"In my city, there are three bridges."
You can just use "ni", but "niwa" puts more of a focus on your city rather than on the existence of the three bridges. It is your city that has three bridges, not another city. When you just use "ni", it's more of a neutral statement about the existence of three bridges in your city.
[Edit: I had a more in-depth conversation about this and did some more research, and it was difficult to come up with a tangible difference between には and に. They both express a general statement about the existence of 3 bridges in your city. You can use the contrastive は as I suggested to say that it's your city, rather than another if maybe you're comparing cities with someone else, but the context would clarify whether you are comparing or if you are just making a general statement. In conclusion, it's complicated but both can be correct.]
That was answered above. で is usually used to indicate where an action is taking place, while に is used to indicate where something exists. -- Hint, you might want to read the questions and answers above before you post your own as there might be an immediate answer to your question. In this case IsolaCiao (above) answered this question a year ago!
To be fair, I don't think Duolingo recognizes any punctuation (other than the occasional apostrophe) or capitalization.
As for whether or not it's needed to be "grammatically correct" in Japanese, I don't think so? In English, we would put one there like "In my city, there are 3 bridges" to separate out the clause. I'm not sure if that's consistent in Japanese as well though.
Is it just me or is this question a bit screwed up? Firstly, when I hover over "city", it only shows the Kanji (街, 市, and 都市) & not the hiragana equivalents / pronunciation like the other words. This would be fine, except the word bank doesn't have any kanji, so there's no way to work it out without looking it up if you don't remember the pronunciation of "city" in Japanese.
Secondly, according to the dictionary I'm using, 街 is pronounced まち, however, it more closely translates to "town; block; neighbourhood; street", and the other Kanji do literally mean city, but don't seem to be pronounced まち (し and とし accordingly).
Did anyone else notice anything like this, or am I mistaken?
You're right that the hints should be in hiragana. On the computer version of duolingo you can report the hints, though I'm not sure if that option is available on the mobile version.
Translating the word "city" into Japanese isn't straightforward. 市 (shi) is a suffix that is attached to city names, e.g. Tokyo is actually called 東京市 (toukyoushi). You don't usually use it on its own. About 都市 (toshi), I can't tell you why but Japanese people do not usually say 私の都市 (watashi no toshi).
街 (machi) is a very natural word to use to talk about where you live. You can see in this hinative answer that when asked to translate "festival in my city", the native Japanese speaker answers "わたしの街のお祭り". While the word doesn't literally mean "city", Japanese people tend to use it in the same way that we use the word "city".
Thanks a lot for that! I've liked using Duolingo so far, but the main drawbacks are (a) the lack of furigana, and (b) there is no proper context or nuance noted within the app itself for cases like these. It makes me glad that the comments are well integrated to the questions and that people are able & willing to spend the time writing up helpful comments like this. :)
I did lodge a report on the hint, so hopefully it's resolved at some point.