Platform 3 was mainly for shuttle service between Stadium and Waterfront during Expo 86 so that attendees could travel to Canada Place without exiting the fairgrounds. The rest of the time now it's mainly used as parking during the off-peak hours and on rare occasion an extra platform for loading/unloading when the regular outbound platform cannot be used or is extremely overloaded.
It can't be used for more regular service due to a complete lack of an escalator or elevator, so it's not compliant with accessibility standards.
Likewise the East entrance was originally an emergency exit. Due to complaints about having to walk up the hill to get into the station from Beatty Street (the stairs and side entrance at the Plaza did not yet exist so you had to walk around the block) it was later converted into an extra entrance, then renovated again to make room for the Compass faregates.
Original photographer unknown
The platform is a bit of a time capsule if you ask me. Signage, seating, lighting...it's virtually unchanged since 1985.
MIPS