"Между прочим" primary is used as "By the way" in english in meaning "Hey, listen, I remembered something interesting...". Literally "I want to say something between another phrases". Specific case of word "прочим" is indicator of this meaning.
On the other case "между прочих" means "between/among another... (some things)". For example "Между прочих коробок стоит коробка от телевизора" - "There is box from TV set among another boxes".
"Среди которых" is "among which / of which". "Three types of computers have been developed: A1, A2 and A3, among which A3 is the most popular machine in the market." - "Было разработано три типа компьютеров: A1, A2 и A3, среди которых A3 - самый популярный на рынке".