Translation:It was three days ago that we arrived.
It's not a passive English sentence; it's a so-called "emphatic" one (which is just to say that it provides a certain emphasis or focus not present in the simple version of the sentence). This particular kind of emphatic structure is called a cleft sentence:
Its appropriateness would depend on the context, but it can be used to elucidate the "是...的" structure:
As for the passive voice, which is completely different, it's explained in the following article, which also touches on the misuse of the term:
The process of the movement of the underlying subject to a place farther back in the sentence, while replacing it with the 'dummy subject' can be referred to as extraposition. The resulting cleft sentence puts emphasis or focus on X in 'It was X, that ....' As others have noted, this is a common way to translate the Chinese 是的 structure.
It's not a passive. To make a passive you need to make the object of the verb the subject and this sentence has no verb with an object. I would say this is in a narrative style like you might read in a novel. So I agree it's a strange word order to pick for a course like this.
Then again, the course assumes we know English and we're studying Chinese. But that doesn't really justify it since it makes a worse match to the Chinese.
The main verb of the sentence is "arrived" and the main subject of the sentence is "it" but the verb is not being performed by the subject. That's the definition of a passive sentence.
This is an atypical example because the active version of the sentence has no object. Therefore, we end up substituting "it".
A similar passive sentence: "He is the one I love."
Passive sentences have their uses, but the translation provided is far from the most natural translation. I also don't see any point in trying to translate "passiveness" between English and Chinese.
You have this wrong. A passive construction turns the object of an active construction into the subject.
Here we have a main clause, "it was three days ago", and a subordinate clause, "that/when we arrived".
"It" is a dummy subject in the main clause, but that's irrelevant. There's no active version that would make any subject here, whether real or a dummy, into an object.
In the subordinate clause, "we" is the subject, and "arrived" is intransitive and takes no object, express or implied. (Note that we shouldn't mistake the subordinate clause for a relative clause. Here "that" is used as a conjunction, and not a relative pronoun or the subject or object of the subordinate clause.)
For comparison, let's look at a sentence that we can make passive. "It was John that she loved" is an example of a similar structure, but where "that" is a relative pronoun and the proper object of the verb in the relative clause. This sentence isn't passive, however. A passive sentence would be "John was loved (by her)". To make our cleft sentence passive, we would have to change it to "it was John that was loved (by her)". Note how "that", previously the object of the relative clause, becomes the subject of the relative clause, and of the verb "was", in the passive version.
A passive sentence needs "X is/was [past participle]", and can take the addition of "by Y". ("To be" can have whatever tense is appropriate, of course.)
The Wikipedia article that I refer to in another comment discusses the misuse of the term "passive". It's worth a read.
No. "Arrived" is not the main verb, it is the verb of the subordinate clause introduced with "that". "Main verb" has a set meaning in grammar and linguistics. The main verb in this sentence is "was".
I believe the point is trying to teach the Chinese passive to English speakers who are already familiar with the passive in their own language. Whoever made this question probably assumed the logical way to do that was to "translate passiveness between English and Chinese".
Perhaps you know better ways to teach this and could consider becoming a contributor to Duolingo's Chinese course. It could certainly use improvement.
Although I agree that they should, it seems to me that "that" is more common than "when" in this construction ("it was X years ago that/when..."), and there's also some dispute over whether both are acceptable or only one or the other. It's possible that someone on the Chinese team believes that "when" is wrong, so I'd say don't hold your breath, though it's also possible that they're just behind in processing reports.
"到" = "arrive"; "來" = "come". The nuance is similar in both languages. "Come" typically implies movement towards the speaker's current location. "Arrive" doesn't. Depending on the context, they may or may not mean essentially the same thing. Here there's no clear context, so you want to stick with the literal translation.
To expand on PeaceJoyPancakes' remark, in English the time frame of reference of the present perfect is the present. It means that as of the time it was said, something has either already occured or not. The past perfect (had arrived) expresses the same idea for a past point in time, but that's not the meaning of Duo's sentence. Neither is another variation with 'since' that does allow us to use the present perfect: It's been three days since we arrived.
And there have been many comments about the difference between 1. "We arrived 3 days ago" and 2. "It was 3 days ago ...," but let me have another go at it, because the majority of comments are from people who still don't get it. The first one (We arrived 3 days ago) is the straight forward, plain vanilla statement, that could also be the answer to the neutral question, "When did you arrive?" The structure of the second sentence (It was ...) emphasizes the piece of information that immediately follows 'It was." When people miss part of what you said, or can't believe what you said, or simply want you to repeat an important point (for a courtroom, for example), they will often ask similarly focused questions using this structure. "When was it that you arrived?" "What was it that you did?" ... District attorneys and police investigators love this kind of question. Storytellers also often use this structure, "It was three days ago that we last heard from our agent in the enemy capital." It's more dramatic than simply "We last heard from our agent ...."