An architect generally designs buildings and other structures, whilst a structural engineer will both design and check that those structures will actually be safe and will remain standing. Although architects do have some engineering knowledge, it is usually left to structural and civil engineers to actually ensure the stability and safety of any design. Architects are good at making buildings look aesthetically pleasing and for designing the finished look.
Quantity surveyors are employed on all sorts of projects and usually get involved once construction starts, in order to ascertain the amount (quantity) of materials etc entailed as construction proceeds, so that the Client can pay the Contractor/Builder.
If you are thinking of a career in any of these fields, why not try to obtain some work experience in all three fields, to see what would really suit you ?