조경원: member of a landscape architecture office
조경가: landscape architect (generic)
조경사: landscape architect (officially certified professional)
As you can see above, ~원 is someone who just happen to be working in that particular field/industry, probably without any kind of qualification whatsoever.
~가 on the other hand implies that this person is quite knowledgeable in that field.
~사 is someone who could be called ~가 holding a governmen issued certificate.
e.g.:
교원 = people working at school, not necessarily teachers
교사 = teacher