I asked this (how to diagnose retained placental fragments) of an
obstetrician the other day, when I ran into a similar problem, though
I would not get too concerned at day three.  There is still lots which
can happen.

The obstetrician said that the best way is ultrasound.  The problem
with ultrasound is you need someone skilled to do it, and
unfortunately, there are a lot of inexperienced technicians and, more
importantly, radiologists doing ultrasound without enough experience
in the field to give a decent reading.  Apparently, serum HCG can also
suggest the diagnosis, in that the HCG should fall to non pregnant
levels within days of delivery.  The trouble is, the obstetrician I
asked also said she had no idea what the level should be, as she never
did HCG after the birth of a baby.  It would be best to check with the
biochemist who runs the lab.

Jack Newman, MD, FRCPC