We use while to talk about two simultaneous actions that go together. We use as to talk about two things that change together in time. I would use while in your example. "While I was waiting at the bus stop, three buses went by in the opposite direction." Because these are two simultaneous actions.

The word waiting, means while is ( probably? )better, because at the same time as you were waiting three buses went by. I think most natives would say this because you used waiting later in the sentence. "while I was WAITING a simultaneous event happened, the bank next door got robbed and the place was swarmed with police" "Just as I got to the bus stop three buses WENT by" This = at the same time as you arrived at the bus stop the other event of three buses going by (that went by) happened. You can use while or as, but in a different format to explain what happened. https://dictionary.cambridge.org/dictionary/english/while one definition of as is the same as while. 1. used to indicate that something happens during the time when something else is taking place. It is just a case of being exposed to the common collocations to know when natives use one and not the other. "while I was waiting ---> "as I was waiting ---> you are free to use either you are explaining that while/during/at the same time three buses went/passed by. I would not argue nor correct either way, it is just being pedantic in my view. Possibly in a classroom setting things like this get over analysed/ analyzed.

Both are correct.

Woody Guthrie, an American folksong writer, wrote the following use of "as": As I went walking that ribbon of highway I saw above me that endless skyway I saw below me the golden valley This land was made for you and me In the movie, The Wizard of Oz, we find a slightly different use of the word "while." The song is "If I Only Had A Brain": I could while away the hours Conferrin' with the flowers, Consulting with the rain; And my head I'd be a scratchin'

While is correctly. Two events happening at the same time. As use when one longer activity interrupted of another short activity.