I think this is what you are after:

select d.name, d.packsize, doc.doctorname, doc.location

from Doctor_Prescribed_Drugs dpd

left join Doctor doc on dpd.doctorref = doc.doctorref

left join Drugs d on dpd.drugref = d.drugref

where dpd.drugref in ('A1', 'B2', 'C3')

If you compare your query with the above query, you will see the problem right away! By using the Doctor_Prescribed_Drugs table as the primary table, or linking (my term) table, it makes it easy to see how the left joins are to be constructed.