"It was three days ago that we arrived."
The meaning is the same but the emphasis is different.
我们是三天前到的 the emphasis is on the time of arrival. Imagine a murder happened at your hotel and you are being interrogated by a police officer when you have started to be there.
我们三天前到了 the emphasis is on the state of having arrived; "3 days ago" is only given as an additional information. Imagine telling a friend who invited you to her wedding in another country that you have already arrived.
Grammatically, time placed at the very beginning is a contextual background of the whole sentence, whereas time placed immediately before the verb is an adverbial phrase. There is no difference in meaning.
I give you an example:
30 years ago, all shops close at 5pm.
With this sentence you can translate like this:
However it would be a little awkward to translate like this:
I am not saying it is wrong, but it would be confusing as 5pm is more describing the closing action, rather than on the phenomenon the whole sentence is about.
I don't think so?? "以前" Means "before", but is usually put at the front of the sentence and when you are certain of the amount of time, whereas "前" is usually put after a certain time. For example, "以前，我有隻狗" meaning "I had a dog before," and "三年前，我有隻狗" meaning "I had a dog 3 years ago." Sorry if it's not clear, I'm kinda bad at explaining