To start with I think it would be a good idea to take your niece to see your GP, as soon as possible.

The doctor can determine whether there is anything that suggests an underlying physical problem contributing to your nieces problem. Also keep fluids up as to not make her dehidrated.

If the cause of the diareha can be established then hopefully the doctor can help you find ways of solving the problem, and if your niece needs to be referred on to a hospital paediatrician or a specialist with experience of dealing with behavioural problems, the doctor will be able to arrange this for you.

I'm sure that there is a simple answer to this problem, as you said she is feeding well which is a good sign that she isn't unwell.

Sometimes with young infants if they have only just stopped passing machonium them they will use there bowels alot as to clear out any nastys from there system.
 

If this baby is breatfeed. Breast milk is a natural laxative and can help with digestion. 

Hope this helps. 

Sounds like everything is right on target for a 3 week old.  The baby's body isn't as good at processing food as our bodies are so that means more dirty diapers when they are newborns.  Which also means they need to eat more frequently (because of that, AND the fact that their stomachs are so tiny).  It will get less frequent as time goes on.

sounds like baby is doing everything normal for a brestfed baby, their stools tend to be looser than that of formula fed.

with #3 as a newborn it seemed like i was changing his nappy all the time i would change when he woke coz he had pood and then again b4 he went back to bed as usually their would be another.

i would say if she is concerned to go and see her doctor.

Newborns have very loose, often watery nappies, particularly when they are breastfed, so diarrhoea is probably unlikely.  This will change as the milk changes.  If bubs is formula fed, then would be more concerning as formula poos are a lot more solid, more like peanut butter or even plastacine.

Newborns tend to need lots of feeds to begin with.  Usually about every 2-4 hours.  The gap between feeds is actually measured from the start of a feed to the start of the next feed.  Means you don't get as much time to yourself as you'd like, but that's a newborn for you!  The feeding patterns can take upto 6 months to full establish.

To me, this little 3 weeker is normal.  I bet your friend finds there are other times of the day when bubs goes longer between feeds.