Dear Heather,
I breastfed my adopted daughter, and also had just three weeks notice.  I
used a manual pump several times a day (5 or 10 minutes), and also used
manual stimulation (more comfy than the hard plastic pump) during the 3
weeks.  By the time she was born, I already had a few drops!  She was 1 1/2
days old when we started nursing (with SNS), and within a few weeks, I
could squirt milk across the room!  We nursed almost constantly, which
built up my supply nicely ( a much slower process without the benefit of
pregnancy hormonal preparation).  I never had a complete supply, but did
often nurse without the SNS, particularly at night, and felt that most of
what she was getting came from me.  Nutritionally, I supported lactation
with a plentiful diet, very abundant fluids, and herbal teas (Red
Raspberry, a few others).  It was much more demanding in many ways than
nursing my homegrown daughter (born almost 2 years later), but well worth
every bit of effort.  I would gladly do it again.  By the way, I then
tandem nursed until my first daughter was 3 1/2.  Aside from being
constantly hungry, it was great!
I am very happy to answer any questions.

Barrett Dunn
Semi-retired midwife, happy mom, and graduate student in Public Health
(annoying all of my professors with my constant emphasis on breastfeeding
issues)