when saying something e.g. three bikes were stolen. is it better fo say "tres bicis fueron robadas" or "fueron robadas tres bicis". is one more formal than the other? thanks
Word order in Spanish follows a principle that is (as far as I know) almost universally common: the topic or theme goes first (what you are going to talk about, especially if it is a new subject you are introducing), and the focus or rheme goes last (the things you want to say about the theme, which is new information). This interacts with the default word order subject-verb-(object).
“Tres bicis fueron robadas” is the default. You introduce the new topic (you say you're going to speak about three bikes) and then inform something about them (that they were stolen). This is an objective, neutral, general statement about an event having to do with three bikes.
“Fueron robadas tres bicis” is a bit different. You jump to the action: something was stolen. So the topic is actually that there was a robbery, and the thing you want to inform is that the things that were robbed were three bikes.
The difference is slight, so both sentences are fairly interchangeable. But in Spanish passive voice, the subject tends to be moved to the end, to the focus position. This might have to do with the fact that the subject of a passive voice sentence is actually a promoted direct object. So it is a subject now, but it does not feel like a subject and does not have the relevance of a subject as topic; typically it is a patient (something that receives or experiences an action, something that is passively done something to). Spanish does not like passive things first in the sentence. (That is why we also tend to say “Se derrumbó el castillo” instead of “El castillo se derrumbó” and the like.)